fix(manifest): lexical pre-check for record_existing + add error-case tests

Address Copilot review (2026-05-11, review id 4266902103):

1. `record_existing` was calling `(self.project_root / rel).is_symlink()`
   BEFORE validating containment. For absolute paths or paths containing
   `..`, this performed a filesystem stat outside the project root before
   `_validate_rel_path()` raised. Add a cheap lexical pre-check that
   delegates to `_validate_rel_path()` for the canonical error messages,
   so the symlink stat only ever runs on paths that are already lexically
   inside the project root.

2. Add focused unit tests in `tests/integrations/test_manifest.py` for
   the symlink and non-regular-file error paths, including:
     - symlink target rejection
     - dangling symlink rejection (caught by the symlink guard before
       the is_file check)
     - directory path rejection (is_file == False)
     - missing-path rejection (is_file == False)
     - absolute-path lexical pre-check
   The Copilot reviewer noted these guards had no focused coverage in
   `test_manifest.py`, only via the `test_integration_claude.py`
   regression test.

3. The third Copilot finding (repeated `dict(self._files)` copies via
   `manifest.files` in the skip branches) is already resolved on this
   branch by using `prior_hashes` — the function-scope snapshot taken at
   the top of `install_shared_infra` — for the membership check, instead
   of `manifest.files`.

AI disclosure: drafted with assistance from Claude (Opus 4.7).

src/specify_cli/integrations/manifest.py

@@ -157,6 +157,15 @@ class IntegrationManifest:
                 treat the entry as permanently broken.
         """
         rel = Path(rel_path)
+        # Cheap lexical pre-check first so absolute / parent-traversal paths
+        # don't trigger a filesystem stat outside the project root before
+        # ``_validate_rel_path`` raises. ``_validate_rel_path`` produces the
+        # canonical error messages used elsewhere.
+        if rel.is_absolute() or ".." in rel.parts:
+            _validate_rel_path(rel, self.project_root)
+            # Defensive: _validate_rel_path always raises on these inputs,
+            # but make the contract explicit if it is ever loosened.
+            raise ValueError(f"Manifest path escapes project root: {rel}")
         # Check ``is_symlink()`` on the un-resolved path because
         # ``_validate_rel_path`` resolves the path (which would follow
         # the symlink and silently record the target instead).