fix(ui): preserve scroll position across comment add/edit/delete

[amix]

Context

Adding, editing, or deleting a comment in the review pane currently lets the diff content shift under the user's eye — inserting an inline comment card above the viewport pushes code down by the card's height, and deleting one pulls everything up. Watcher-triggered reloads of .dunk/comments.json have the same problem.

What was changed

• Generalized the existing layout/wrap anchor restoration useLayoutEffect in DiffPane to also fire on files identity changes (previousFilesRef.current !== files). One source of truth for viewport preservation across every relayout-causing input.
• For files-only changes, when the natural anchor cannot resolve in the new geometry (e.g. the inline-note row was just deleted), fall back to the nearest surviving diff row before clamping. Inline-note stable keys are inline-note:${annotation.id}, so an in-place comment edit still resolves cleanly and keeps the reader's position inside the card.
• DiffSectionRowBounds carries a kind ("diff-row" | "inline-note") so anchor pickers can prefer survivable rows.
• findViewportRowAnchor accepts { preferDiffRows?: boolean }; resolveViewportRowAnchorTop now returns number | null so callers can apply a clamp fallback instead of scrolling to file body top.

Validation

• bun run typecheck, bun test (334 pass), bun run test:integration (21 pass — full existing PTY suite, including the layout/wrap scroll-preservation tests that exercise the same scaffolding this change extends), bun run lint, bun run format.
• Manual TTY smoke run on a real diff.
• 5 new unit tests in viewportAnchor.test.ts covering: preferDiffRows row-picking, inline-note anchor surviving an in-place edit (with explicit height-bump assertion), inline-note anchor returning null after deletion, diff-row fallback resolving cleanly after deletion, and missing-file resolver behavior.

Expert review

Design was validated before implementation via expert-letter to Codex (.expert/letter-20260511T104143Z-*.md). The unified-effect approach, preferDiffRows bias, nullable resolver contract, and clamp fallback are all from that response.

Out of scope

• Drive-by extraction of the inline VisibleAgentNote id-format mapping from DiffPane. The same string format is built in two places, but the existing site is unrelated to this change.
• Dedicated comment-mutation request-id signal threaded from App to DiffPane. Codex explicitly advised against App-level plumbing here; the files identity trigger is the intended design and also benefits filter/reload paths.
• Pane-level PTY integration test exercising comment add + scroll preservation end-to-end. The new helper-level tests cover the algorithmic core, and the DiffPane wiring reuses the layout/wrap restoration scaffolding already exercised by existing PTY coverage. Worth a follow-up.

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