[P3.4] "View Diff" affordance shows a placeholder, not a diff

[frankbria]

Severity: medium | Category: ux | Phase: P3.4

Problem

The execution stream's "View Diff" toggle expands to only the one-line event.message (self-marked placeholder in a comment), advertising a capability it cannot deliver.

Evidence

web-ui/src/components/execution/FileChangeEvent.tsx:41

Acceptance criteria

Wire the panel to the real diff (correlate following OutputEvents / fetch file diff) or remove the button until the data source exists.

Dependencies

None

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_{Filed from the SaaS launch-readiness audit. Atomic: one developer, one session. Work order: strictly P0.1 → P3.12 (no forward dependencies).}

[coderabbitai]

🔗 Related PRs

#339 - feat: Execution Monitor View with real-time SSE streaming (#332) [merged]
#497 - feat(web-ui): compress and summarize verbose execution event stream (#475) [merged]
#522 - feat(review): add business context (task/requirement) to diff viewer file tree (#480) [merged]

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