Reduce chat timeline work-row rerenders

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What Changed

• Removed the list-wide timeline activity context that every simple work row consumed.
• Added turnInProgress to derived work-row data so only rows belonging to the currently unsettled turn receive changed render props.
• Kept revert-button disabled state in a narrow context consumed only by revert controls.
• Moved touched timeline effects into named hooks to keep component bodies render-focused.
• Added row-derivation tests proving inactive work rows are structurally reused when a different turn settles.

Why

React Doctor identified chat/timeline performance hotspots, including ChatView and MessagesTimeline compiler/rerender concerns. Manual inspection found TimelineRowActivityCtx broadcasting active-turn state to every SimpleWorkEntryRow, causing long timelines to rerender old work rows whenever the current turn toggled running/settled. Row-scoping that state preserves the existing structural-sharing strategy and narrows updates to rows whose visual state can actually change.

UI Changes

No intentional UI appearance changes. React Scan before/after recordings were captured with a temporary uncommitted harness rendering the real MessagesTimeline and toggling active-turn state:

• Before: react_scan_before_timeline_context_rerenders.mp4
• After: react_scan_after_row_scoped_rerenders.mp4

The after recording shows rerender highlighting scoped away from inactive work rows compared with the before recording.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Validation:

• npx -y react-doctor@latest apps/web --verbose --category performance (reported broader existing performance findings; used to guide hotspot selection)
• pnpm --dir apps/web exec vp test run --passWithNoTests --project unit src/components/chat/MessagesTimeline.logic.test.ts src/components/chat/MessagesTimeline.test.tsx -> 38 tests passed
• NODE_OPTIONS="--experimental-strip-types" pnpm exec vp check -> passed with 0 errors / 20 pre-existing warnings
• NODE_OPTIONS="--experimental-strip-types" pnpm exec vp run typecheck -> passed

  

Note

Reduce chat timeline work-row rerenders by computing turnInProgress per work row

• Moves the activeTurnInProgress global flag out of MessagesTimeline props and instead computes a turnInProgress boolean on each work row in MessagesTimeline.logic.ts, set only when the row contains entries from the currently unsettled turn.
• isRowUnchanged now checks turnInProgress equality so stable rows are reused across renders when a turn settles, avoiding unnecessary rerenders of prior work rows.
• SimpleWorkEntryRow reads its own turnInProgress prop instead of a shared context, so only rows belonging to the active turn render as in-progress.
• Replaces TimelineRowActivityCtx with a narrower TimelineRevertControlsCtx carrying a single disabled flag for RevertUserMessageButton.
• Several inline effects in MessagesTimeline are extracted into dedicated hooks (useFoldToggleSettlingReset, useLatestTurnFoldState, useInitialTimelineEndScroll, useWorkingTimerText, usePreserveWorkGroupAnchorOnExpand) to isolate side-effects.

^{Macroscope summarized 9b74066.}