Phase 5: Replace derived-index fields with memoized hooks

[jennings]

Replace derived-index fields on the store with hooks that memoize over React Query data + local UI state.

Fields removed from store

• viewedFilenames
• commentRangeLookup
• commentAnchorLookup
• navigableItems

Note: viewedFiles (the persisted set of user-marked-viewed filenames) stays on the store. Only the derived viewedFilenames (that resolves against current file SHAs) moves.

New hooks

• useViewedFilenames() — useMemo over useFiles() result + store.viewedFiles.
• useCommentRangeLookup() — useMemo over useComments() + useReviewThreads() + useFiles().
• useCommentAnchorLookup() — useMemo over the same inputs.
• useNavigableItems() — useMemo over useFiles() + useComments() + store.pendingComments.

Extracted pure functions

Move computeViewedFilenames, computeCommentRangeLookup, computeCommentAnchorLookup, computeNavigableItems out of the store class and into src/browser/lib/pr-review-derived.ts as pure exports. Hooks call these directly.

Consumer changes

• conversations-sidebar.tsx, pr-review.tsx, useKeyboardNavigation.ts, useCommentActions.ts, useCurrentFileComments.ts — switch from usePRReviewSelector(s => s.commentRangeLookup) etc. to the new hooks.

Tests

• Port existing tests for compute* functions directly to the new pure exports (same inputs/outputs; no wrapping).
• Test each hook wires inputs correctly under a QueryClientProvider + PRReviewProvider.
• End-to-end style: navigating with keyboard shortcuts (which reads navigableItems) still works.

Part of #351

Depends on #353.