The default json watch stream emits one enveloped JSON object per line via
PrintNdjson, but the fetch-message failure branch used PrintJson (multi-line
MarshalIndent) and bypassed the envelope, so a mid-stream failure injected a
multi-line {ok:false,...} blob that breaks line-based NDJSON parsers. Route
failures through watchFailureOutputValue: bare payload for --format data,
identity-tagged single line for the default json envelope.
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