file-safety (S4b follow-up): guard diff-view saveChanges() user-accept path with observation/version check

[easonLiangWorldedtech]

Scope

Follow-up from the S4b review finding on #1408 (epic #1375, file-safety series): when focus-disruption prevention is disabled, the diff-view approval path saves via DiffViewProvider.saveChanges() without an observation/version check. A file that changes after the diff preview was rendered can be silently overwritten on user accept.

The guarded-write wiring in this series covers the tool-driven write paths (write_to_file / edit via guardedWrite() with the observation registry + version tokens). The user-accept path in the diff view is the remaining save path that bypasses the guard.

Proposed behavior

1. When a diff preview is rendered, register an observation for the target (read bytes → version token), reusing the existing observation-registry semantics from S4a/S4b.
2. On user accept, saveChanges() must verify the recorded version against the current file state before writing:
   • match → proceed with the (guarded) save;
   • mismatch or target deleted → reject with a GuardRejectedError-class outcome and surface a re-review prompt in the diff view (stale content highlighted), instead of overwriting.
3. Behavior when focus-disruption prevention is enabled stays as-is (that path already refuses to save across a focus change).

Acceptance criteria

• Regression test: file modified after preview render, user accepts → no overwrite, re-review prompt shown.
• Regression test: unmodified file → accept saves as before.
• No behavior change for tool-driven writes (covered by the S4a/S4b suites).

References

• feat(tools): wire guarded writes into the diff-view save paths (S4b, #1375) Zoo-Code-Org/Zoo-Code#1408 (S4b diff-view wiring) — finding (b), deliberately deferred from that PR.
• [EPIC] File Write Safety Prevent Concurrent Write Races Data Corruption Zoo-Code-Org/Zoo-Code#1375 (epic) — S4 file-safety series.
• S4a commit 7a25fc076 — replaceIfVersion ENOENT → GuardRejectedError normalization (the error-shaping precedent for the mismatch case above).