fix(ui): navigate comments by nearest stream position

[amix]

Context

Backport of modem-dev/hunk#315. PR 1 of a series bringing over genuinely-missing upstream fixes (most of hunk's good work is already independently in dunk).

}/{ (next/previous comment) from a hunk that has no comment of its own snapped to the first or last comment in the entire review instead of the nearest one in review-stream order. Root cause: findNextHunkCursor fell back to cursors[0] / cursors[len-1] whenever the current hunk was absent from the annotated subset.

What was changed

• findNextHunkCursor gains an optional streamCursors arg (defaults to cursors, so existing callers are a behavioral no-op) plus a findNearestCursorIndex helper that locates the current {fileId,hunkIndex} in full review-stream order and picks the nearest annotated cursor in the travel direction. Non-cyclic — clamps to the nearest edge, matching existing hunk navigation.
• The full hunkCursors stream is threaded into moveToAnnotatedHunk (useReviewController), the only runtime call site. buildReviewStream derives hunkCursors and annotatedHunkCursors from the same visibleFiles atomically, so the stream and the annotated subset are always consistent.
• Adapted to dunk's signature (precomputed cursorIndex Map) and on-disk comments model — not a copy of upstream.

Expert-reviewed (advice incorporated: added the controller-level regression test, the empty-annotated assertion, and the stream-order invariant comment).

Verification

• bun run typecheck, bun run lint: clean.
• hunks.test.ts + useReviewController.test.tsx: 14 pass. New unit cases cover nearest-in-stream selection (both directions), non-cyclic clamp, and empty-annotated → null. New controller-level test exercises moveToAnnotatedHunk from an unannotated middle hunk and asserts it lands on the nearest annotated hunk (not first/last) — this is the regression at the level the behavior lives.
• No PTY test: the fix is deterministic cursor math with no rendering/timing dimension and there is no existing PTY comment-nav harness; the controller test is the correct "where behavior lives" coverage per CLAUDE.md.

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