refactor(web): drop the unreachable paint-only thread timeline path#25787

resolveThreadSwitchTimeline can no longer return another thread's timeline, so the whole paint-only/held-timeline path in the chat view is unreachable. This deletes it. Behavior is unchanged.

Why it is dead

On main (32bf48f428), every return in resolveThreadSwitchTimeline sets displayThreadKey: input.activeThreadKey (lines 440, 446, 449), and the lastReady parameter (line 437) is never read. isPaintOnlyThreadTimeline(displayThreadKey, activeThreadKey) (L423-L430) is therefore always false, so paintOnlyDisplayedTimeline in ChatView is constant false and every branch keyed on it never runs. The fallback to the last-viewed thread most likely went away with #21560 (79b14831cb).

What was removed

• ChatView's own resolveThreadSwitchTimeline call. Only its displayThreadKey was read, and that always equals activeThreadKey, so displayedTimelineKey is now activeThreadKey ?? routeThreadKey.
• isPaintOnlyThreadTimeline, paintOnlyDisplayedTimeline, and the ~30 ternaries/spreads on it in ChatView's ThreadTimeline props, plus the one on threadFindEnabled. Each prop now gets the value its false branch already gave it.
• heldPaintContext, peekHeldThreadTimeline, lastReadyThreadKey, the lastReady param, and HeldThreadTimeline.markdownCwd / .workspaceRoot.
• The noopHeld* and EMPTY_HELD_TURN_DIFF_SUMMARIES constants.
• MessagesTimeline's paintOnly prop, useTimelineRewind's paintOnlyDisplayedTimeline input, and ThreadTimeline's readyMarkdownCwd / readyWorkspaceRoot props, which only fed the held context.
• threadKeysShareEnvironment, which only a test used.
• resolveThreadSwitchTimeline now returns the entries directly instead of { entries, displayThreadKey }.

What stays

• ThreadTimeline's useLayoutEffect that remembers the ready timeline is still live. peekRememberedThreadTimeline reads it so that switching back to a cached thread paints that thread's own entries while its detail resyncs, instead of an empty list.
• That same-thread fallback in resolveThreadSwitchTimeline and its test are kept. The cross-environment test, which only passed lastReady, now checks that a destination with nothing remembered never paints the last-viewed thread.

Nothing outside apps/web imports any of these, including desktop, mobile, and client-runtime.

Verification

• vp run check:changed: format, lint, knip, typecheck, and tests all pass.
• ChatView.logic.test.ts (43 tests) and the MessagesTimeline browser, logic, and active-tools tests (117 tests) pass. No useTimelineRewind tests exist.

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