Add UnifiedDiffView tests for the diff annotation surface

[selfcontained]

Recurring Test Enforcer run. The local suite was green on first pass (23rd consecutive run), so this PR is coverage only — no product code changed.

Local validation

Lane	Result
pnpm run check	✅
pnpm run test	✅ server 150 files / 2605 tests (8 skip), web 734 → 765, ext 60, scripts 4
pnpm run test:e2e	✅ 178 passed / 12 skipped
pnpm run test:e2e:live	✅ 11 passed in 18.5s, zero leaked e2e-* tmux sessions (14th consecutive)
pnpm run finalize:web	✅
CI scan (gh run list --limit 50)	✅ 50/50 success, no cross-branch flakes

Both entries on the flake list are recognition-only records of already-fixed infra problems; neither recurred. The server-tests-isolated.sh wrapper from #922 provisioned its own Postgres and left nothing behind, and the three orphaned dispatch-postgres-test-* containers noted last run are gone.

What this adds

apps/web/src/components/app/unified-diff-view.test.tsx — 31 tests covering two modules that had no direct coverage:

• use-diff-widgets.tsx — builds the changeKey → ReactElement map of inline annotations (review feedback, draft comments, the inline comment form).
• unified-diff-view.tsx — renders the diff, owns gutter-click line-selection semantics, and feeds the widget map to react-diff-view.

Between them they decide where every review annotation lands, which is exactly the kind of wiring a stubbed test would miss. So the tests mount the real tree — react-diff-view, the real hook, and the real InlineFeedbackAnnotation / InlineDraftAnnotation / InlineCommentForm — with only the HTTP seam and the animation layer mocked. Rows are located by their code text and widgets by row adjacency, so no change keys are hardcoded and a mis-keyed widget cannot pass.

Pinned behavior:

• widget anchoring to the last changed line in a range (multi-line and single-line)
• feedback skip rules — no start line (even with an end line set, where ln >= null would otherwise coerce to ln >= 0), and ranges covering no rendered change
• multiple feedback items sharing an anchor grouped into one widget; distinct anchors kept separate
• drafts composing under feedback on a shared anchor rather than replacing it
• the three-way gating of the comment form (selection + agentId + commentOpen) and its cancel / add-draft / start-review callbacks, including comment trimming and the whitespace-only no-op
• gutter selection: single click, shift-extend below and above the anchor, click-inside-to-clear, and the delete-line no-op
• selected-gutter marking and the floating comment affordance's show/hide/click contract

Mutation battery: 25/25 behavior-changing mutants killed across both source files.

Follow-up filed, not fixed here

DIS-168 — opening the inline comment form hides any feedback or draft annotation anchored to the same line. The draft pass explicitly composes with what is already at a key; the comment-form pass assigns unconditionally and clobbers it. The mechanical fix is one line, but whether the form should take over the row is a UX call, so current behavior is pinned with a comment saying to invert that assertion once it is decided.

Also noted while writing these: parseDiff returns one hunk-less file even for junk input, so the view's "Unable to parse diff" fallback only fires if parseDiff throws. What an unparseable payload actually renders is an empty table — pinned as-is with a comment.

Review

One code-review persona pass (review 651): 3 items, all test hygiene, all resolved.

• restored the scrollIntoView prototype patch via its captured property descriptor (vi.spyOn can't be used — jsdom never defines the method, so there is nothing to wrap)
• the floating-affordance lookup now throws when more than one direct-child button exists, so a future sibling button fails loudly instead of silently returning the wrong element
• widget adjacency kept strict on purpose, with the rationale written down: immediate succession is how react-diff-view expresses "anchored to this line", and a nearby-row scan would let a widget anchored one line off still pass

🤖 Generated with Claude Code