ChatView subscribed to the live thread projection, so every streamed
assistant or reasoning delta re-rendered it and rebuilt the uncached
timeline region.

client-runtime gains quietThreadAtom and quietVisibleTurnItemsAtom. They
hold the previous value while the only change is text streamed into
existing assistant and reasoning items. ChatView reads the thread through
them. ThreadTimeline reads the live turn items and derives the rows it
renders, and the find provider reads the live projection to refresh open
results.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>