Add JobExecutionView state container

The DAP debugger needs to map a runtime IStep back to a source line
when answering `stackTrace` requests. The renderer (#PR1b) produces
the YAML and per-entry start lines from an immutable list, but the
debugger's view grows over the job's lifetime: post steps register
lazily, and the integration layer needs O(1) IStep -> line lookup
at every pause.

This commit adds JobExecutionView, a stateful append-only wrapper
around the renderer. It maintains:
  - the current entry list,
  - the most recent rendered YAML,
  - a Dictionary<IStep, int> for fast line lookup.

Each Append can register an entry in one of three modes:
  - with a stepIdentity: registers the IStep -> line mapping
    immediately;
  - with a matchKey: registers an unclaimed placeholder that a
    later TryClaim binds to a real IStep (used when an entry is
    predicted before the runner materializes its IStep, e.g. a
    Post-step placeholder synthesized at job-init from an action's
    metadata);
  - with neither: a static informational entry that needs no line
    lookup.

This is part 3 of 5. The DAP-integration PR that consumes this
container is the final follow-up.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Runner.Worker/Dap/JobExecutionView.cs

@@ -0,0 +1,276 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Sdk;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Stateful, append-only container that wraps <see cref="JobExecutionViewRenderer"/>
+    /// for runtime use. Maintains a mutable list of entries, caches the rendered YAML,
+    /// and provides O(1) lookup from <see cref="IStep"/> identity to the current line
+    /// in the rendered YAML where that step's <c>- step:</c> key appears.
+    ///
+    /// Each <see cref="Append"/> can register the entry in one of three modes:
+    ///   - With a non-null <c>stepIdentity</c>: registers the IStep→line mapping
+    ///     immediately. Used for entries whose real <see cref="IStep"/> is already
+    ///     known at append time.
+    ///   - With a non-null <c>matchKey</c>: registers an unclaimed placeholder
+    ///     that a later <see cref="TryClaim"/> binds to a real <see cref="IStep"/>.
+    ///     Used for entries whose <see cref="IStep"/> is materialized later. A
+    ///     placeholder that is never claimed simply stays in the view and is never
+    ///     paused on — the IStep→line mapping is only populated on claim.
+    ///   - With neither: a static entry that needs no line lookup.
+    ///
+    /// <see cref="Append"/> and <see cref="AppendRange"/> never remove or reorder
+    /// existing entries. <see cref="TryClaim"/> does not re-render. The IStep→line
+    /// mapping is rebuilt on every render, so lookups stay accurate even if a later
+    /// Append happens to shift previously-emitted entries.
+    /// </summary>
+    internal sealed class JobExecutionView
+    {
+        private readonly object _lock = new();
+        private readonly string _jobId;
+        private readonly List<JobExecutionViewEntry> _entries = new();
+        private readonly List<IStep> _stepIdentities = new();
+        private readonly Dictionary<IStep, int> _lineByStep =
+            new(ReferenceEqualityComparer.Instance);
+        // Map matchKey -> entry index for placeholders awaiting a future
+        // TryClaim. Removed when claimed.
+        private readonly Dictionary<string, int> _unclaimedByKey =
+            new(StringComparer.Ordinal);
+        private string _yaml;
+        private IReadOnlyList<int> _entryStartLines = Array.Empty<int>();
+
+        public JobExecutionView(string jobId)
+        {
+            if (string.IsNullOrWhiteSpace(jobId))
+            {
+                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
+            }
+
+            _jobId = jobId;
+            Render();
+        }
+
+        public string JobId
+        {
+            get { return _jobId; }
+        }
+
+        /// <summary>
+        /// Currently rendered YAML. Always reflects all entries appended so far,
+        /// plus the synthetic Setup header and Cleanup footer emitted by the renderer.
+        /// </summary>
+        public string Yaml
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _yaml;
+                }
+            }
+        }
+
+        /// <summary>Number of entries (excludes synthetic Setup/Cleanup boundaries).</summary>
+        public int EntryCount
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _entries.Count;
+                }
+            }
+        }
+
+        /// <summary>
+        /// 1-based line where entry <paramref name="entryIndex"/>'s <c>- step:</c> key
+        /// currently appears in <see cref="Yaml"/>.
+        /// </summary>
+        public int GetLine(int entryIndex)
+        {
+            lock (_lock)
+            {
+                if (entryIndex < 0 || entryIndex >= _entries.Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(entryIndex));
+                }
+
+                return _entryStartLines[entryIndex];
+            }
+        }
+
+        /// <summary>
+        /// 1-based line for the entry whose <see cref="IStep"/> reference identity
+        /// matches <paramref name="step"/>. Returns null if <paramref name="step"/>
+        /// is null or has not been registered.
+        /// </summary>
+        public int? TryGetLineForStep(IStep step)
+        {
+            if (step == null)
+            {
+                return null;
+            }
+
+            lock (_lock)
+            {
+                if (_lineByStep.TryGetValue(step, out var line))
+                {
+                    return line;
+                }
+
+                return null;
+            }
+        }
+
+        /// <summary>
+        /// Append a new entry. Exactly one of <paramref name="stepIdentity"/>
+        /// or <paramref name="matchKey"/> may be non-null (or both may be
+        /// null for a static entry that needs no line lookup):
+        ///   - <paramref name="stepIdentity"/> non-null: registers the
+        ///     IStep→line mapping immediately. Use when the real
+        ///     <see cref="IStep"/> is known at append time.
+        ///   - <paramref name="matchKey"/> non-null: registers an unclaimed
+        ///     placeholder that a later <see cref="TryClaim"/> binds to a
+        ///     real <see cref="IStep"/>.
+        /// Re-renders the YAML and updates the start-line table.
+        /// </summary>
+        /// <returns>1-based line number of the newly-appended entry's <c>- step:</c> key.</returns>
+        public int Append(JobExecutionViewEntry entry, IStep stepIdentity = null, string matchKey = null)
+        {
+            ArgUtil.NotNull(entry, nameof(entry));
+            if (stepIdentity != null && matchKey != null)
+            {
+                throw new ArgumentException(
+                    "Append cannot register both a step identity and a placeholder match key on the same entry; pass at most one.");
+            }
+
+            lock (_lock)
+            {
+                if (stepIdentity != null && _lineByStep.ContainsKey(stepIdentity))
+                {
+                    throw new InvalidOperationException("step already registered in execution view");
+                }
+                if (matchKey != null && _unclaimedByKey.ContainsKey(matchKey))
+                {
+                    throw new InvalidOperationException($"matchKey already registered: {matchKey}");
+                }
+
+                _entries.Add(entry);
+                _stepIdentities.Add(stepIdentity);
+                Render();
+
+                int index = _entries.Count - 1;
+                if (matchKey != null)
+                {
+                    _unclaimedByKey[matchKey] = index;
+                }
+                return _entryStartLines[index];
+            }
+        }
+
+        /// <summary>
+        /// Bind a previously-appended placeholder entry (registered via
+        /// <see cref="Append(JobExecutionViewEntry, IStep, string)"/> with
+        /// a non-null <c>matchKey</c>) to a real <see cref="IStep"/>.
+        /// Returns the 1-based line of the now-claimed entry on success.
+        /// Returns null when no unclaimed placeholder exists for
+        /// <paramref name="matchKey"/>, OR when <paramref name="stepIdentity"/>
+        /// is already registered for a different entry (defensive).
+        /// Does not re-render: claim only updates the IStep -> line index.
+        /// </summary>
+        public int? TryClaim(string matchKey, IStep stepIdentity)
+        {
+            if (matchKey == null)
+            {
+                throw new ArgumentNullException(nameof(matchKey));
+            }
+            if (stepIdentity == null)
+            {
+                throw new ArgumentNullException(nameof(stepIdentity));
+            }
+
+            lock (_lock)
+            {
+                if (!_unclaimedByKey.TryGetValue(matchKey, out int index))
+                {
+                    return null;
+                }
+                if (_lineByStep.ContainsKey(stepIdentity))
+                {
+                    // Bail rather than double-register the step.
+                    return null;
+                }
+
+                _unclaimedByKey.Remove(matchKey);
+                _stepIdentities[index] = stepIdentity;
+                _lineByStep[stepIdentity] = _entryStartLines[index];
+                return _entryStartLines[index];
+            }
+        }
+
+        /// <summary>
+        /// Bulk-append for the initial population. Equivalent to calling
+        /// <see cref="Append"/> once per pair, but renders only once at the end.
+        /// State is left unchanged if any input is invalid.
+        /// </summary>
+        public void AppendRange(IEnumerable<(JobExecutionViewEntry entry, IStep stepIdentity)> items)
+        {
+            ArgUtil.NotNull(items, nameof(items));
+
+            // Materialize first so we don't enumerate twice.
+            var materialized = new List<(JobExecutionViewEntry entry, IStep stepIdentity)>(items);
+            for (int i = 0; i < materialized.Count; i++)
+            {
+                if (materialized[i].entry == null)
+                {
+                    throw new ArgumentException($"items[{i}].entry is null.", nameof(items));
+                }
+            }
+
+            lock (_lock)
+            {
+                // Validate no duplicates within the input or with existing identities,
+                // before mutating state.
+                var seen = new HashSet<IStep>(ReferenceEqualityComparer.Instance);
+                foreach (var (_, stepIdentity) in materialized)
+                {
+                    if (stepIdentity == null)
+                    {
+                        continue;
+                    }
+                    if (_lineByStep.ContainsKey(stepIdentity) || !seen.Add(stepIdentity))
+                    {
+                        throw new InvalidOperationException("step already registered in execution view");
+                    }
+                }
+
+                foreach (var (entry, stepIdentity) in materialized)
+                {
+                    _entries.Add(entry);
+                    _stepIdentities.Add(stepIdentity);
+                }
+                Render();
+            }
+        }
+
+        // Caller MUST hold _lock (constructor's call is safe — no concurrent access yet).
+        private void Render()
+        {
+            var result = JobExecutionViewRenderer.Render(_jobId, _entries.AsReadOnly());
+            _yaml = result.Yaml;
+            _entryStartLines = result.EntryStartLines;
+
+            _lineByStep.Clear();
+            for (int i = 0; i < _stepIdentities.Count; i++)
+            {
+                var step = _stepIdentities[i];
+                if (step != null)
+                {
+                    _lineByStep[step] = _entryStartLines[i];
+                }
+            }
+        }
+    }
+}

src/Runner.Worker/Dap/JobExecutionViewRenderer.cs

@@ -0,0 +1,336 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Text;
+using GitHub.Runner.Sdk;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Phase a step occupies in the runner's flat execution sequence.
+    /// Setup and Cleanup are NOT modeled here — they are synthetic
+    /// boundaries hard-coded by <see cref="JobExecutionViewRenderer"/>
+    /// and cannot be constructed by callers.
+    /// </summary>
+    internal enum JobExecutionPhase
+    {
+        Pre,
+        Main,
+        Post,
+    }
+
+    /// <summary>
+    /// One step in the rendered execution view. Pure data; no link to
+    /// any worker type. Phase 2 will translate runner step objects
+    /// into instances of this record.
+    /// </summary>
+    internal sealed class JobExecutionViewEntry
+    {
+        public JobExecutionViewEntry(
+            JobExecutionPhase phase,
+            string displayName,
+            string uses = null,
+            string run = null,
+            string sourcePath = null,
+            int sourceLine = 0,
+            string id = null,
+            string @if = null,
+            string continueOnError = null,
+            string timeoutMinutes = null,
+            string envYaml = null,
+            string withYaml = null,
+            string shell = null,
+            string workingDirectory = null)
+        {
+            if (string.IsNullOrWhiteSpace(displayName))
+            {
+                throw new ArgumentException("displayName must not be null or whitespace.", nameof(displayName));
+            }
+            if (sourcePath != null && sourceLine < 1)
+            {
+                throw new ArgumentException(
+                    "sourceLine must be >= 1 when sourcePath is provided.",
+                    nameof(sourceLine));
+            }
+
+            Phase = phase;
+            DisplayName = displayName;
+            Uses = uses;
+            Run = run;
+            SourcePath = sourcePath;
+            SourceLine = sourceLine;
+            Id = id;
+            If = @if;
+            ContinueOnError = continueOnError;
+            TimeoutMinutes = timeoutMinutes;
+            EnvYaml = envYaml;
+            WithYaml = withYaml;
+            Shell = shell;
+            WorkingDirectory = workingDirectory;
+        }
+
+        public JobExecutionPhase Phase { get; }
+        public string DisplayName { get; }
+        public string Uses { get; }
+        public string Run { get; }
+        public string SourcePath { get; }
+        public int SourceLine { get; }
+        public string Id { get; }
+        public string If { get; }
+        public string ContinueOnError { get; }
+        public string TimeoutMinutes { get; }
+        // Pre-serialized YAML fragment, already indented for embedding
+        // under the entry's `env:` key (6-space child indent).
+        public string EnvYaml { get; }
+        public string WithYaml { get; }
+        public string Shell { get; }
+        public string WorkingDirectory { get; }
+    }
+
+    /// <summary>
+    /// Output of <see cref="JobExecutionViewRenderer.Render"/>: the YAML
+    /// document plus a parallel array of 1-based line numbers, one per
+    /// input entry, where each entry's <c>- step:</c> key appears.
+    /// Synthetic Setup/Cleanup boundaries are not tracked here.
+    /// </summary>
+    internal readonly struct RenderResult
+    {
+        public RenderResult(string yaml, IReadOnlyList<int> entryStartLines)
+        {
+            Yaml = yaml;
+            EntryStartLines = entryStartLines;
+        }
+
+        public string Yaml { get; }
+        public IReadOnlyList<int> EntryStartLines { get; }
+    }
+
+    /// <summary>
+    /// Renders a job's execution-view YAML. Pure function; no I/O,
+    /// no logging, no static state. Output format and Setup/Cleanup
+    /// boundaries are fixed; callers cannot influence them.
+    ///
+    /// Output is structured as phase-keyed top-level sections:
+    /// <c>setup:</c>, <c>pre:</c>, <c>main:</c>, <c>post:</c>, <c>cleanup:</c>.
+    /// <c>setup:</c> and <c>cleanup:</c> always render; <c>pre:</c>,
+    /// <c>main:</c>, <c>post:</c> only render when they contain at least
+    /// one entry.
+    /// </summary>
+    internal static class JobExecutionViewRenderer
+    {
+        public static RenderResult Render(string jobId, IReadOnlyList<JobExecutionViewEntry> entries)
+        {
+            if (string.IsNullOrWhiteSpace(jobId))
+            {
+                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
+            }
+            ArgUtil.NotNull(entries, nameof(entries));
+
+            // Pre-validate non-null entries before any output, so partial
+            // state is never observed by callers.
+            for (int i = 0; i < entries.Count; i++)
+            {
+                if (entries[i] == null)
+                {
+                    throw new ArgumentException($"entries[{i}] is null.", nameof(entries));
+                }
+            }
+
+            var sb = new StringBuilder();
+            var startLines = new int[entries.Count];
+            int newlinesEmitted = 0;
+
+            // Header (3 lines).
+            sb.Append("# Job: ").Append(YamlScalarFormatter.Format(jobId)).Append('\n');
+            sb.Append("# Runner execution plan — read-only.\n");
+            sb.Append('\n');
+            newlinesEmitted += 3;
+
+            // setup: section — always present.
+            sb.Append("setup:\n");
+            sb.Append("  - step: Setup job\n");
+            newlinesEmitted += 2;
+
+            // Render phase sections in fixed order. Each emits a leading
+            // blank line separator before its header.
+            EmitPhaseSection(sb, "pre", JobExecutionPhase.Pre, entries, startLines, ref newlinesEmitted);
+            EmitPhaseSection(sb, "main", JobExecutionPhase.Main, entries, startLines, ref newlinesEmitted);
+            EmitPhaseSection(sb, "post", JobExecutionPhase.Post, entries, startLines, ref newlinesEmitted);
+
+            // cleanup: section — always present, preceded by a blank line.
+            sb.Append('\n');
+            sb.Append("cleanup:\n");
+            sb.Append("  - step: Complete job\n");
+
+            return new RenderResult(sb.ToString(), Array.AsReadOnly(startLines));
+        }
+
+        private static void EmitPhaseSection(
+            StringBuilder sb,
+            string sectionName,
+            JobExecutionPhase phase,
+            IReadOnlyList<JobExecutionViewEntry> entries,
+            int[] startLines,
+            ref int newlinesEmitted)
+        {
+            // Skip the section entirely if no entries belong to this phase.
+            bool any = false;
+            for (int i = 0; i < entries.Count; i++)
+            {
+                if (entries[i].Phase == phase) { any = true; break; }
+            }
+            if (!any)
+            {
+                return;
+            }
+
+            // Blank line separator + section header.
+            sb.Append('\n');
+            sb.Append(sectionName).Append(":\n");
+            newlinesEmitted += 2;
+
+            for (int i = 0; i < entries.Count; i++)
+            {
+                var entry = entries[i];
+                if (entry.Phase != phase)
+                {
+                    continue;
+                }
+
+                // 1-based line of the `- step:` key for this entry.
+                startLines[i] = newlinesEmitted + 1;
+
+                sb.Append("  - step: ").Append(YamlScalarFormatter.Format(entry.DisplayName));
+                sb.Append('\n');
+                newlinesEmitted++;
+
+                switch (phase)
+                {
+                    case JobExecutionPhase.Pre:
+                    case JobExecutionPhase.Post:
+                        if (!string.IsNullOrEmpty(entry.Uses))
+                        {
+                            sb.Append("    action: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        // No source: annotation for pre/post.
+                        break;
+
+                    case JobExecutionPhase.Main:
+                        if (!string.IsNullOrEmpty(entry.Id))
+                        {
+                            sb.Append("    id: ").Append(YamlScalarFormatter.Format(entry.Id)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Uses))
+                        {
+                            sb.Append("    uses: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Run))
+                        {
+                            if (entry.Run.IndexOf('\n') < 0)
+                            {
+                                sb.Append("    run: ").Append(YamlScalarFormatter.Format(entry.Run)).Append('\n');
+                                newlinesEmitted++;
+                            }
+                            else
+                            {
+                                sb.Append("    run: |\n");
+                                newlinesEmitted++;
+                                newlinesEmitted += AppendIndentedBlock(sb, entry.Run, "      ");
+                            }
+                        }
+                        if (!string.IsNullOrEmpty(entry.If))
+                        {
+                            sb.Append("    if: ").Append(YamlScalarFormatter.Format(entry.If)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.ContinueOnError))
+                        {
+                            sb.Append("    continue-on-error: ").Append(entry.ContinueOnError).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.TimeoutMinutes))
+                        {
+                            sb.Append("    timeout-minutes: ").Append(entry.TimeoutMinutes).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.EnvYaml))
+                        {
+                            sb.Append("    env:\n");
+                            newlinesEmitted++;
+                            sb.Append(entry.EnvYaml).Append('\n');
+                            newlinesEmitted += CountChar(entry.EnvYaml, '\n') + 1;
+                        }
+                        if (!string.IsNullOrEmpty(entry.WithYaml))
+                        {
+                            sb.Append("    with:\n");
+                            newlinesEmitted++;
+                            sb.Append(entry.WithYaml).Append('\n');
+                            newlinesEmitted += CountChar(entry.WithYaml, '\n') + 1;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Shell))
+                        {
+                            sb.Append("    shell: ").Append(YamlScalarFormatter.Format(entry.Shell)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.WorkingDirectory))
+                        {
+                            sb.Append("    working-directory: ").Append(YamlScalarFormatter.Format(entry.WorkingDirectory)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (entry.SourcePath != null)
+                        {
+                            sb.Append("    source: ")
+                              .Append(entry.SourcePath)
+                              .Append(':')
+                              .Append(entry.SourceLine.ToString(CultureInfo.InvariantCulture))
+                              .Append('\n');
+                            newlinesEmitted++;
+                        }
+                        break;
+                }
+            }
+        }
+
+        private static int AppendIndentedBlock(StringBuilder sb, string text, string indent)
+        {
+            int newlines = 0;
+            int i = 0;
+            while (i < text.Length)
+            {
+                int end = text.IndexOf('\n', i);
+                int lineEnd = end < 0 ? text.Length : end;
+                int trimEnd = lineEnd;
+                if (trimEnd > i && text[trimEnd - 1] == '\r')
+                {
+                    trimEnd--;
+                }
+                if (trimEnd > i)
+                {
+                    sb.Append(indent);
+                    sb.Append(text, i, trimEnd - i);
+                }
+                sb.Append('\n');
+                newlines++;
+                if (end < 0)
+                {
+                    break;
+                }
+                i = end + 1;
+            }
+            return newlines;
+        }
+
+        private static int CountChar(string s, char c)
+        {
+            int n = 0;
+            for (int i = 0; i < s.Length; i++)
+            {
+                if (s[i] == c) n++;
+            }
+            return n;
+        }
+    }
+}

src/Runner.Worker/Dap/YamlScalarFormatter.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Globalization;
+using System.IO;
+using GitHub.Runner.Sdk;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Formats a single string as a quote-safe YAML scalar by routing it
+    /// through YamlDotNet's <see cref="Emitter"/>. The returned text is
+    /// safe to splice into a hand-emitted YAML document fragment.
+    ///
+    /// Caller responsibility: this only handles the scalar value; it does
+    /// not emit a key, indent, or trailing newline.
+    /// </summary>
+    internal static class YamlScalarFormatter
+    {
+        /// <summary>
+        /// Return <paramref name="value"/> formatted as a YAML scalar:
+        /// plain, single-quoted, or double-quoted as the emitter chooses,
+        /// with no surrounding document markers or trailing newline.
+        /// </summary>
+        public static string Format(string value)
+        {
+            ArgUtil.NotNull(value, nameof(value));
+
+            using var sw = new StringWriter(CultureInfo.InvariantCulture);
+            // Force LF line breaks; YamlDotNet's Emitter calls WriteLine,
+            // which would otherwise produce CRLF on Windows and break
+            // both our document-end stripping below and downstream
+            // consumers that assume a single line-break convention.
+            sw.NewLine = "\n";
+            var emitter = new Emitter(sw);
+            emitter.Emit(new StreamStart());
+            emitter.Emit(new DocumentStart(null, null, true));
+            emitter.Emit(new Scalar(null, null, value, ScalarStyle.Any, true, true));
+            emitter.Emit(new DocumentEnd(true));
+            emitter.Emit(new StreamEnd());
+
+            string raw = sw.ToString();
+            // Strip YAML document markers. Emitter elides these for most
+            // scalars but emits "--- " (with space) for some edge cases
+            // (e.g. empty strings). Defensively handle "---\n" too.
+            if (raw.StartsWith("--- ", StringComparison.Ordinal))
+            {
+                raw = raw.Substring(4);
+            }
+            else if (raw.StartsWith("---\n", StringComparison.Ordinal))
+            {
+                raw = raw.Substring(4);
+            }
+            raw = raw.TrimEnd('\n');
+            const string DocEndMarker = "\n...";
+            if (raw.EndsWith(DocEndMarker, StringComparison.Ordinal))
+            {
+                raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
+            }
+            return raw;
+        }
+    }
+}

src/Test/L0/Worker/JobExecutionViewL0.cs

@@ -0,0 +1,442 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Dap;
+using Moq;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class JobExecutionViewL0
+    {
+        private static JobExecutionViewEntry MainEntry(string name)
+        {
+            return new JobExecutionViewEntry(JobExecutionPhase.Main, name, run: name);
+        }
+
+        private static IStep NewStep(string displayName = "step")
+        {
+            var mock = new Mock<IStep>();
+            mock.Setup(s => s.DisplayName).Returns(displayName);
+            return mock.Object;
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Constructor_RendersEmptyView()
+        {
+            var view = new JobExecutionView("my-job");
+
+            Assert.Equal(0, view.EntryCount);
+            Assert.Contains("# Job: my-job", view.Yaml);
+            Assert.Contains("- step: Setup job", view.Yaml);
+            Assert.Contains("- step: Complete job", view.Yaml);
+
+            // Only the two synthetic boundaries appear.
+            int stepCount = view.Yaml.Split("- step: ").Length - 1;
+            Assert.Equal(2, stepCount);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(null)]
+        [InlineData("")]
+        [InlineData("   ")]
+        public void Constructor_ThrowsOnInvalidJobId(string jobId)
+        {
+            Assert.Throws<ArgumentException>(() => new JobExecutionView(jobId));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_IncrementsEntryCount()
+        {
+            var view = new JobExecutionView("j");
+
+            int line0 = view.Append(MainEntry("a"));
+            int line1 = view.Append(MainEntry("b"));
+            int line2 = view.Append(MainEntry("c"));
+
+            Assert.Equal(3, view.EntryCount);
+            Assert.True(line0 < line1);
+            Assert.True(line1 < line2);
+            Assert.Equal(line0, view.GetLine(0));
+            Assert.Equal(line1, view.GetLine(1));
+            Assert.Equal(line2, view.GetLine(2));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_PreservesPriorEntryLines()
+        {
+            var view = new JobExecutionView("j");
+
+            int l0 = view.Append(MainEntry("a"));
+            int l1 = view.Append(MainEntry("b"));
+            int l2 = view.Append(MainEntry("c"));
+
+            view.Append(MainEntry("d"));
+            Assert.Equal(l0, view.GetLine(0));
+            Assert.Equal(l1, view.GetLine(1));
+            Assert.Equal(l2, view.GetLine(2));
+
+            view.Append(MainEntry("e"));
+            Assert.Equal(l0, view.GetLine(0));
+            Assert.Equal(l1, view.GetLine(1));
+            Assert.Equal(l2, view.GetLine(2));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_RegistersStepIdentity()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            int line = view.Append(MainEntry("a"), step);
+
+            Assert.Equal(line, view.GetLine(0));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_NullStepIdentity_StillAppends()
+        {
+            var view = new JobExecutionView("j");
+
+            view.Append(MainEntry("a"), stepIdentity: null);
+
+            Assert.Equal(1, view.EntryCount);
+            Assert.Null(view.TryGetLineForStep(null));
+            Assert.Contains("- step: a", view.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_DuplicateStepIdentity_Throws()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            view.Append(MainEntry("a"), step);
+            Assert.Throws<InvalidOperationException>(() => view.Append(MainEntry("b"), step));
+
+            // State preserved: only the first entry is present.
+            Assert.Equal(1, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_NullEntry_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.Append(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_AppendsAllAndRendersOnce()
+        {
+            var view = new JobExecutionView("j");
+            var steps = Enumerable.Range(0, 5).Select(i => NewStep("s" + i)).ToList();
+            var items = steps
+                .Select((s, i) => (entry: MainEntry("e" + i), stepIdentity: s))
+                .ToList();
+
+            view.AppendRange(items);
+
+            Assert.Equal(5, view.EntryCount);
+            for (int i = 0; i < 5; i++)
+            {
+                int line = view.GetLine(i);
+                Assert.Equal(line, view.TryGetLineForStep(steps[i]));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_RejectsDuplicateInInput()
+        {
+            var view = new JobExecutionView("j");
+            var dup = NewStep();
+            var items = new List<(JobExecutionViewEntry, IStep)>
+            {
+                (MainEntry("a"), dup),
+                (MainEntry("b"), dup),
+            };
+
+            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
+            Assert.Equal(0, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_RejectsOverlapWithExisting()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            view.Append(MainEntry("a"), step);
+
+            var items = new List<(JobExecutionViewEntry, IStep)>
+            {
+                (MainEntry("b"), step),
+            };
+
+            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
+            Assert.Equal(1, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_NullItems_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.AppendRange(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryGetLineForStep_NullStep_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Null(view.TryGetLineForStep(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryGetLineForStep_UnknownStep_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            Assert.Null(view.TryGetLineForStep(step));
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(-1)]
+        [InlineData(2)]
+        public void GetLine_OutOfRange_Throws(int index)
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"));
+            view.Append(MainEntry("b"));
+
+            Assert.Throws<ArgumentOutOfRangeException>(() => view.GetLine(index));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Yaml_UpdatesAfterAppend()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("first"));
+            string before = view.Yaml;
+            Assert.Contains("- step: first", before);
+
+            view.Append(MainEntry("second"));
+            string after = view.Yaml;
+
+            Assert.Contains("- step: first", after);
+            Assert.Contains("- step: second", after);
+            Assert.NotEqual(before, after);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Yaml_AlwaysEndsWithCleanupBoundary()
+        {
+            var view = new JobExecutionView("j");
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+
+            view.Append(MainEntry("a"));
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+
+            view.Append(MainEntry("b"));
+            view.Append(MainEntry("c"));
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_WithMatchKey_TracksUnclaimed()
+        {
+            var view = new JobExecutionView("j");
+
+            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
+
+            var step = NewStep("real");
+            int? claimed = view.TryClaim("k1", step);
+            Assert.Equal(line, claimed);
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_UnknownKey_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            Assert.Null(view.TryClaim("nope", NewStep()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_AlreadyClaimed_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            var first = NewStep("first");
+            Assert.NotNull(view.TryClaim("k1", first));
+
+            var second = NewStep("second");
+            Assert.Null(view.TryClaim("k1", second));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_StepAlreadyRegistered_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            // Step is registered for the first entry.
+            view.Append(MainEntry("a"), step);
+            // A placeholder is registered for the second entry.
+            view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1");
+
+            // Trying to claim the placeholder with the already-registered
+            // step must return null (defensive — would otherwise double-bind).
+            Assert.Null(view.TryClaim("k1", step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_DuplicateMatchKey_Throws()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            Assert.Throws<InvalidOperationException>(
+                () => view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_MatchKeyNull_BehavesLikeOldOverload()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            int line = view.Append(MainEntry("a"), step);
+
+            Assert.Equal(line, view.GetLine(0));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+            // TryClaim with any key must return null since no matchKey was registered.
+            Assert.Null(view.TryClaim("anything", NewStep()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_AfterClaim_TryGetLineForStepResolves()
+        {
+            var view = new JobExecutionView("j");
+            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
+
+            var step = NewStep();
+            Assert.Equal(line, view.TryClaim("k1", step));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+
+            // And a later Append doesn't lose the claim (Render rebuilds
+            // the IStep -> line map from the persisted identities).
+            view.Append(MainEntry("b"));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_NullArgs_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.TryClaim(null, NewStep()));
+            Assert.Throws<ArgumentNullException>(() => view.TryClaim("k", null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ConcurrentAppends_DontCorruptState()
+        {
+            var view = new JobExecutionView("j");
+            const int N = 50;
+            var steps = Enumerable.Range(0, N).Select(i => NewStep("s" + i)).ToList();
+            var returnedLines = new ConcurrentBag<int>();
+
+            var tasks = Enumerable.Range(0, N).Select(i => Task.Run(() =>
+            {
+                int line = view.Append(MainEntry("e" + i), steps[i]);
+                returnedLines.Add(line);
+            })).ToArray();
+
+            await Task.WhenAll(tasks);
+
+            Assert.Equal(N, view.EntryCount);
+            Assert.Equal(N, returnedLines.Distinct().Count());
+
+            // Every step identity resolves to some line in [0, N).
+            var entryLines = Enumerable.Range(0, N).Select(view.GetLine).ToHashSet();
+            Assert.Equal(N, entryLines.Count);
+            foreach (var step in steps)
+            {
+                int? line = view.TryGetLineForStep(step);
+                Assert.NotNull(line);
+                Assert.Contains(line.Value, entryLines);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_RejectsBothStepIdentityAndMatchKey()
+        {
+            // Allowing both would orphan the IStep→line mapping the moment
+            // TryClaim overwrites _stepIdentities[index] for a different
+            // step, so the API rejects the combination at append time.
+            var view = new JobExecutionView("j");
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
+            Assert.Throws<ArgumentException>(() =>
+                view.Append(entry, stepIdentity: NewStep("real"), matchKey: "k1"));
+            // State unchanged.
+            Assert.Equal(0, view.EntryCount);
+        }
+    }
+}

src/Test/L0/Worker/JobExecutionViewRendererL0.cs

@@ -0,0 +1,598 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Worker.Dap;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class JobExecutionViewRendererL0
+    {
+        // Verbatim expected YAML for the design doc's "Worked example".
+        // The render output is structured as phase-keyed top-level sections;
+        // there is no per-entry `phase:` field. The setup: and cleanup:
+        // sections always render; pre:/main:/post: render only when
+        // they contain at least one entry. The Main entries surface
+        // user-authored step parameters pre-evaluation (no expression
+        // substitution); Pre/Post entries stay minimal.
+        private const string ExpectedWorkedExampleYaml =
+            "# Job: build\n" +
+            "# Runner execution plan — read-only.\n" +
+            "\n" +
+            "setup:\n" +
+            "  - step: Setup job\n" +
+            "\n" +
+            "pre:\n" +
+            "  - step: Pre actions/checkout@v4\n" +
+            "    action: actions/checkout@v4\n" +
+            "  - step: Pre actions/cache@v5\n" +
+            "    action: actions/cache@v5\n" +
+            "\n" +
+            "main:\n" +
+            "  - step: actions/checkout@v4\n" +
+            "    uses: actions/checkout@v4\n" +
+            "    source: .github/workflows/ci.yml:10\n" +
+            "  - step: Cache Primes\n" +
+            "    id: cache-primes\n" +
+            "    uses: actions/cache@v5\n" +
+            "    with:\n" +
+            "      path: prime-numbers\n" +
+            "      key: ${{ runner.os }}-primes\n" +
+            "    source: .github/workflows/ci.yml:12\n" +
+            "  - step: Run tests\n" +
+            "    id: test\n" +
+            "    run: |\n" +
+            "      echo starting\n" +
+            "      npm test\n" +
+            "    if: ${{ github.event_name == 'push' }}\n" +
+            "    env:\n" +
+            "      NODE_ENV: production\n" +
+            "    shell: bash\n" +
+            "    working-directory: ./api\n" +
+            "    source: .github/workflows/ci.yml:18\n" +
+            "  - step: npm ci\n" +
+            "    run: npm ci\n" +
+            "    source: .github/workflows/ci.yml:28\n" +
+            "\n" +
+            "post:\n" +
+            "  - step: Post actions/cache@v5\n" +
+            "    action: actions/cache@v5\n" +
+            "  - step: Post actions/checkout@v4\n" +
+            "    action: actions/checkout@v4\n" +
+            "\n" +
+            "cleanup:\n" +
+            "  - step: Complete job\n";
+
+        private static List<JobExecutionViewEntry> WorkedExampleEntries()
+        {
+            return new List<JobExecutionViewEntry>
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/checkout@v4", uses: "actions/checkout@v4"),
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/cache@v5", uses: "actions/cache@v5"),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "actions/checkout@v4", uses: "actions/checkout@v4", sourcePath: ".github/workflows/ci.yml", sourceLine: 10),
+                new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "Cache Primes",
+                    uses: "actions/cache@v5",
+                    id: "cache-primes",
+                    withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
+                    sourcePath: ".github/workflows/ci.yml",
+                    sourceLine: 12),
+                new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "Run tests",
+                    run: "echo starting\nnpm test",
+                    id: "test",
+                    @if: "${{ github.event_name == 'push' }}",
+                    envYaml: "      NODE_ENV: production",
+                    shell: "bash",
+                    workingDirectory: "./api",
+                    sourcePath: ".github/workflows/ci.yml",
+                    sourceLine: 18),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "npm ci", run: "npm ci", sourcePath: ".github/workflows/ci.yml", sourceLine: 28),
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/cache@v5", uses: "actions/cache@v5"),
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/checkout@v4", uses: "actions/checkout@v4"),
+            };
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_MatchesDesignDocWorkedExample()
+        {
+            var entries = WorkedExampleEntries();
+
+            var result = JobExecutionViewRenderer.Render("build", entries);
+
+            Assert.Equal(ExpectedWorkedExampleYaml, result.Yaml);
+            Assert.Equal(8, result.EntryStartLines.Count);
+            var lines = result.Yaml.Split('\n');
+            for (int i = 0; i < entries.Count; i++)
+            {
+                Assert.StartsWith("  - step: ", lines[result.EntryStartLines[i] - 1]);
+                Assert.Contains(entries[i].DisplayName, lines[result.EntryStartLines[i] - 1]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_AlwaysEmitsSetupAndCleanup()
+        {
+            var result = JobExecutionViewRenderer.Render("job-1", new List<JobExecutionViewEntry>());
+
+            const string expected =
+                "# Job: job-1\n" +
+                "# Runner execution plan — read-only.\n" +
+                "\n" +
+                "setup:\n" +
+                "  - step: Setup job\n" +
+                "\n" +
+                "cleanup:\n" +
+                "  - step: Complete job\n";
+            Assert.Equal(expected, result.Yaml);
+            Assert.Empty(result.EntryStartLines);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsEmptyOptionalSections()
+        {
+            // Only a Main entry — pre:/post: must not appear.
+            var result = JobExecutionViewRenderer.Render("j", new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "echo", run: "echo hello"),
+            });
+
+            Assert.Contains("setup:\n", result.Yaml);
+            Assert.Contains("main:\n", result.Yaml);
+            Assert.Contains("cleanup:\n", result.Yaml);
+            Assert.DoesNotContain("\npre:\n", result.Yaml);
+            Assert.DoesNotContain("\npost:\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsPhaseSectionsInFixedOrder()
+        {
+            // Input order [Post, Pre, Main] should still render as setup → pre → main → post → cleanup.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-a", uses: "a/b@v1"),
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-a", uses: "a/b@v1"),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a/b@v1"),
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            string yaml = result.Yaml;
+
+            int setupIdx = yaml.IndexOf("setup:\n", StringComparison.Ordinal);
+            int preIdx = yaml.IndexOf("\npre:\n", StringComparison.Ordinal);
+            int mainIdx = yaml.IndexOf("\nmain:\n", StringComparison.Ordinal);
+            int postIdx = yaml.IndexOf("\npost:\n", StringComparison.Ordinal);
+            int cleanupIdx = yaml.IndexOf("\ncleanup:\n", StringComparison.Ordinal);
+            Assert.True(setupIdx >= 0 && preIdx > setupIdx && mainIdx > preIdx && postIdx > mainIdx && cleanupIdx > postIdx,
+                $"section ordering wrong: setup={setupIdx} pre={preIdx} main={mainIdx} post={postIdx} cleanup={cleanupIdx}");
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_StartLinesAlignWithInputOrder()
+        {
+            // Input order is [Pre, Main, Post]; output order is also pre/main/post,
+            // but startLines must be indexed by INPUT position, not by section.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-x", uses: "x/y@v1"),     // index 0
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-x", uses: "x/y@v1"),    // index 1
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-x", uses: "x/y@v1"),    // index 2
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            var lines = result.Yaml.Split('\n');
+
+            Assert.StartsWith("  - step: pre-x", lines[result.EntryStartLines[0] - 1]);
+            Assert.StartsWith("  - step: main-x", lines[result.EntryStartLines[1] - 1]);
+            Assert.StartsWith("  - step: post-x", lines[result.EntryStartLines[2] - 1]);
+            // And input-order ordering of start lines is strictly increasing
+            // when phases are in declaration order matching the section order.
+            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[1]);
+            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[2]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_StartLinesFollowInputOrderEvenWhenPhasesAreInterleaved()
+        {
+            // Input order is [Main A, Pre B, Main C]: pre section will render
+            // first (Pre B) and main second (Main A then Main C). startLines
+            // must still be indexed by input order.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a@v1"),  // index 0 — renders in main section
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-b", uses: "b@v1"),    // index 1 — renders in pre section
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-c", uses: "c@v1"),  // index 2 — renders in main section
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            var lines = result.Yaml.Split('\n');
+
+            Assert.StartsWith("  - step: main-a", lines[result.EntryStartLines[0] - 1]);
+            Assert.StartsWith("  - step: pre-b", lines[result.EntryStartLines[1] - 1]);
+            Assert.StartsWith("  - step: main-c", lines[result.EntryStartLines[2] - 1]);
+            // The pre section comes before main: input-index-1 entry's line is
+            // before input-index-0 entry's line.
+            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[0]);
+            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[2]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EntryStartLinesPointAtStepKeys()
+        {
+            var entries = WorkedExampleEntries();
+            var result = JobExecutionViewRenderer.Render("build", entries);
+            var lines = result.Yaml.Split('\n');
+
+            for (int i = 0; i < result.EntryStartLines.Count; i++)
+            {
+                int oneBased = result.EntryStartLines[i];
+                Assert.True(oneBased >= 1 && oneBased <= lines.Length, $"start line {oneBased} out of range");
+                Assert.StartsWith("  - step: ", lines[oneBased - 1]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EntryStartLinesExcludeSetupAndCleanup()
+        {
+            var entries = WorkedExampleEntries();
+            var result = JobExecutionViewRenderer.Render("build", entries);
+            var lines = result.Yaml.Split('\n');
+
+            int setupLine = -1, cleanupLine = -1;
+            for (int i = 0; i < lines.Length; i++)
+            {
+                if (lines[i] == "  - step: Setup job") setupLine = i + 1;
+                if (lines[i] == "  - step: Complete job") cleanupLine = i + 1;
+            }
+            Assert.True(setupLine > 0 && cleanupLine > 0, "Setup/Cleanup lines must exist");
+            Assert.DoesNotContain(setupLine, result.EntryStartLines);
+            Assert.DoesNotContain(cleanupLine, result.EntryStartLines);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData("hello")]
+        [InlineData("with: colon")]
+        [InlineData("with#hash")]
+        [InlineData(" leading")]
+        [InlineData("trailing ")]
+        [InlineData("a\"b")]
+        [InlineData("a\\b")]
+        [InlineData("@at")]
+        [InlineData("*star")]
+        public void Render_QuotesSpecialChars(string displayName)
+        {
+            // Round-trip the rendered YAML through YamlDotNet's deserializer
+            // and assert the parsed step's display name matches the input.
+            // This decouples the test from any specific quoting style.
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, displayName);
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            var deserializer = new YamlDotNet.Serialization.DeserializerBuilder().Build();
+            var doc = deserializer.Deserialize<Dictionary<string, List<Dictionary<string, object>>>>(result.Yaml);
+            Assert.NotNull(doc);
+            Assert.True(doc.ContainsKey("main"), "rendered YAML missing top-level 'main' key");
+            var mainSteps = doc["main"];
+            Assert.Single(mainSteps);
+            Assert.Equal(displayName, mainSteps[0]["step"] as string);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsSourceAnnotationForMainStep()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "npm ci",
+                run: "npm ci",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 42);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    source: .github/workflows/ci.yml:42\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsSourceAnnotationForPreAndPost()
+        {
+            var pre = new JobExecutionViewEntry(
+                JobExecutionPhase.Pre,
+                "Pre actions/checkout@v4",
+                uses: "actions/checkout@v4",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 9);
+            var post = new JobExecutionViewEntry(
+                JobExecutionPhase.Post,
+                "Post actions/checkout@v4",
+                uses: "actions/checkout@v4",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 9);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
+
+            Assert.DoesNotContain("source:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsMultilineRunAsBlockScalar()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "multi",
+                run: "echo a\necho b\necho c");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    run: |\n", result.Yaml);
+            Assert.Contains("      echo a\n", result.Yaml);
+            Assert.Contains("      echo b\n", result.Yaml);
+            Assert.Contains("      echo c\n", result.Yaml);
+            Assert.DoesNotContain("truncated", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsAllUserAuthoredParamsForActionStep()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Run action",
+                uses: "actions/cache@v5",
+                id: "cache-primes",
+                @if: "${{ github.event_name == 'push' }}",
+                continueOnError: "true",
+                timeoutMinutes: "10",
+                envYaml: "      NODE_ENV: production",
+                withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
+                sourcePath: "ci.yml",
+                sourceLine: 5);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    id: cache-primes\n", result.Yaml);
+            Assert.Contains("    uses: actions/cache@v5\n", result.Yaml);
+            Assert.Contains("    continue-on-error: true\n", result.Yaml);
+            Assert.Contains("    timeout-minutes: 10\n", result.Yaml);
+            Assert.Contains("    env:\n      NODE_ENV: production\n", result.Yaml);
+            Assert.Contains("    with:\n      path: prime-numbers\n      key: ${{ runner.os }}-primes\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsRunStepWithShellAndWorkingDirectory()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Run tests",
+                run: "echo starting\nnpm test",
+                id: "test",
+                shell: "bash",
+                workingDirectory: "./api");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    run: |\n      echo starting\n      npm test\n", result.Yaml);
+            Assert.Contains("    shell: bash\n", result.Yaml);
+            Assert.Contains("    working-directory: ./api\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_PreservesExpressionsInRenderedYaml()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Cache",
+                uses: "actions/cache@v5",
+                withYaml: "      key: ${{ runner.os }}-primes");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            // Expressions render exactly as authored — no evaluation.
+            Assert.Contains("${{ runner.os }}-primes", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_PrePostStepsRemainMinimal()
+        {
+            // Even if a pre/post entry carries user-param fields (it shouldn't
+            // in production, but the renderer must defensively drop them),
+            // only step: + action: render for these phases.
+            var pre = new JobExecutionViewEntry(
+                JobExecutionPhase.Pre,
+                "Pre actions/cache@v5",
+                uses: "actions/cache@v5",
+                id: "should-not-appear",
+                envYaml: "      X: y",
+                withYaml: "      key: nope");
+            var post = new JobExecutionViewEntry(
+                JobExecutionPhase.Post,
+                "Post actions/cache@v5",
+                uses: "actions/cache@v5",
+                id: "should-not-appear",
+                envYaml: "      X: y");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
+
+            Assert.DoesNotContain("id:", result.Yaml);
+            Assert.DoesNotContain("env:", result.Yaml);
+            Assert.DoesNotContain("with:", result.Yaml);
+            Assert.DoesNotContain("should-not-appear", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_FieldOrderIsStable()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Everything",
+                uses: "actions/cache@v5",
+                id: "x",
+                @if: "always()",
+                continueOnError: "false",
+                timeoutMinutes: "5",
+                envYaml: "      A: 1",
+                withYaml: "      key: k",
+                sourcePath: "ci.yml",
+                sourceLine: 1);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+            var y = result.Yaml;
+            int iStep = y.IndexOf("    - step: ", StringComparison.Ordinal) >= 0
+                ? y.IndexOf("- step:", StringComparison.Ordinal) : y.IndexOf("- step:", StringComparison.Ordinal);
+            int iId = y.IndexOf("    id:", StringComparison.Ordinal);
+            int iUses = y.IndexOf("    uses:", StringComparison.Ordinal);
+            int iIf = y.IndexOf("    if:", StringComparison.Ordinal);
+            int iCoe = y.IndexOf("    continue-on-error:", StringComparison.Ordinal);
+            int iTm = y.IndexOf("    timeout-minutes:", StringComparison.Ordinal);
+            int iEnv = y.IndexOf("    env:", StringComparison.Ordinal);
+            int iWith = y.IndexOf("    with:", StringComparison.Ordinal);
+            int iSrc = y.IndexOf("    source:", StringComparison.Ordinal);
+            Assert.True(iId < iUses && iUses < iIf && iIf < iCoe && iCoe < iTm && iTm < iEnv && iEnv < iWith && iWith < iSrc,
+                $"order wrong: id={iId} uses={iUses} if={iIf} coe={iCoe} tm={iTm} env={iEnv} with={iWith} src={iSrc}");
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsEmptyOptionalFields()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "bare",
+                uses: "a/b@v1");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+            Assert.DoesNotContain("    id:", result.Yaml);
+            Assert.DoesNotContain("    if:", result.Yaml);
+            Assert.DoesNotContain("    continue-on-error:", result.Yaml);
+            Assert.DoesNotContain("    timeout-minutes:", result.Yaml);
+            Assert.DoesNotContain("    env:", result.Yaml);
+            Assert.DoesNotContain("    with:", result.Yaml);
+            Assert.DoesNotContain("    shell:", result.Yaml);
+            Assert.DoesNotContain("    working-directory:", result.Yaml);
+            Assert.DoesNotContain("    source:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_HandlesEmptyEntries()
+        {
+            var result = JobExecutionViewRenderer.Render("j", new List<JobExecutionViewEntry>());
+
+            Assert.Empty(result.EntryStartLines);
+            Assert.Contains("  - step: Setup job\n", result.Yaml);
+            Assert.Contains("  - step: Complete job\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_NoPerEntryPhaseField()
+        {
+            // The phase: <value> per-entry field is gone — the section
+            // header is the phase indicator. Guard against accidental
+            // regressions.
+            var result = JobExecutionViewRenderer.Render("build", WorkedExampleEntries());
+            Assert.DoesNotContain("phase:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnNullJobId()
+        {
+            Assert.Throws<ArgumentException>(
+                () => JobExecutionViewRenderer.Render(null, new List<JobExecutionViewEntry>()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnWhitespaceJobId()
+        {
+            Assert.Throws<ArgumentException>(
+                () => JobExecutionViewRenderer.Render("   ", new List<JobExecutionViewEntry>()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnNullEntries()
+        {
+            Assert.Throws<ArgumentNullException>(
+                () => JobExecutionViewRenderer.Render("j", null));
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(null, 1)]
+        [InlineData("", 1)]
+        [InlineData("   ", 1)]
+        public void Entry_Constructor_RejectsBadDisplayName(string displayName, int sourceLine)
+        {
+            Assert.Throws<ArgumentException>(
+                () => new JobExecutionViewEntry(JobExecutionPhase.Main, displayName, sourceLine: sourceLine));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Entry_Constructor_RejectsZeroLineWhenSourcePathSet()
+        {
+            Assert.Throws<ArgumentException>(
+                () => new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "ok",
+                    sourcePath: "ci.yml",
+                    sourceLine: 0));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_AlwaysUsesLfLineBreaks()
+        {
+            // Regression: YamlDotNet's Emitter calls WriteLine, which on
+            // Windows produces CRLF (the host's Environment.NewLine).
+            // The renderer's hand-emitted skeleton always uses '\n'; this
+            // test asserts the scalar formatter doesn't sneak CRLF in.
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, "with: colon", id: "step-1", uses: "actions/checkout@v4");
+            var result = JobExecutionViewRenderer.Render("job-1", new[] { entry });
+            Assert.DoesNotContain("\r", result.Yaml);
+        }
+    }
+}

src/Test/L0/Worker/YamlScalarFormatterL0.cs

@@ -0,0 +1,119 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Worker.Dap;
+using Xunit;
+using YamlDotNet.Serialization;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class YamlScalarFormatterL0
+    {
+        private static readonly IDeserializer Deserializer = new DeserializerBuilder().Build();
+
+        // Embed the formatter output inside a minimal YAML mapping and
+        // round-trip through YamlDotNet, asserting the parsed value equals
+        // the original input. Decouples assertions from the emitter's
+        // quoting choices (plain vs single- vs double-quoted).
+        private static void AssertRoundTrips(string value)
+        {
+            string scalar = YamlScalarFormatter.Format(value);
+            string yaml = $"k: {scalar}\n";
+
+            Dictionary<string, object> doc;
+            try
+            {
+                doc = Deserializer.Deserialize<Dictionary<string, object>>(yaml);
+            }
+            catch (Exception ex)
+            {
+                throw new Xunit.Sdk.XunitException(
+                    $"Formatted scalar did not round-trip as valid YAML.\nInput: '{value}'\nFormatted: '{scalar}'\nFull YAML:\n{yaml}\nError: {ex}");
+            }
+            Assert.NotNull(doc);
+            Assert.True(doc.ContainsKey("k"), $"missing key in parsed doc. Formatted: '{scalar}'");
+            Assert.Equal(value, doc["k"] as string);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData("hello")]
+        [InlineData("with: colon")]
+        [InlineData("with#hash")]
+        [InlineData(" leading")]
+        [InlineData("trailing ")]
+        [InlineData("a\"b")]
+        [InlineData("a\\b")]
+        [InlineData("@at")]
+        [InlineData("*star")]
+        [InlineData("&amp")]
+        [InlineData("?question")]
+        [InlineData("!exclaim")]
+        [InlineData("- dash")]
+        [InlineData("{brace}")]
+        [InlineData("[bracket]")]
+        public void Format_RoundTripsThroughYamlDeserializer(string value)
+        {
+            // The formatter must produce output that, embedded under a key,
+            // parses back to exactly the input. The emitter is free to
+            // pick plain, single-quoted, or double-quoted style.
+            AssertRoundTrips(value);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_PlainAscii_NoQuotingNeeded()
+        {
+            // Sanity check that the simple case stays plain.
+            Assert.Equal("hello", YamlScalarFormatter.Format("hello"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NoTrailingNewline()
+        {
+            Assert.False(YamlScalarFormatter.Format("hello").EndsWith("\n"));
+            Assert.False(YamlScalarFormatter.Format("with: colon").EndsWith("\n"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NoDocumentMarkers()
+        {
+            // The emitter wraps the scalar in a document; the formatter
+            // must strip both `--- ` (with space) and `---\n` (on its
+            // own line) prefixes plus the `\n...` suffix.
+            Assert.DoesNotContain("---", YamlScalarFormatter.Format("hello"));
+            Assert.DoesNotContain("...", YamlScalarFormatter.Format("hello"));
+            // Empty string is one of the cases where the emitter does
+            // produce a document marker by default.
+            Assert.DoesNotContain("---", YamlScalarFormatter.Format(""));
+            Assert.DoesNotContain("...", YamlScalarFormatter.Format(""));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_AlwaysUsesLfLineBreaks()
+        {
+            // Regression: YamlDotNet's Emitter calls WriteLine, which on
+            // Windows produces CRLF (the host's Environment.NewLine).
+            // Format must force LF so the output round-trips regardless
+            // of platform.
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format("hello"));
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format("with: colon"));
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format(""));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NullValue_Throws()
+        {
+            Assert.Throws<ArgumentNullException>(() => YamlScalarFormatter.Format(null));
+        }
+    }
+}