Fix chat scroll jitter during streaming; respect reduced motion

[kateebonner]

Why

The chat visibly jitters/bounces while a response streams, and occasionally jumps on its own even when idle.

Root cause: the message timeline had three independent systems all writing scrollTop to the bottom on the same content update, each reading scroll geometry at a slightly different instant from a virtua list whose item heights are estimates. When virtua corrects a measured height a frame later, the writers have already landed on slightly different positions → visible bounce.

What changed (3 files)

1. Gate auto-scroll on real streaming state — packages/app/src/pages/session.tsx
createAutoScroll({ working: () => true }) → real sync.data.session_working(id). The auto-scroller's ResizeObserver was force-following the bottom on any reflow (image load, tool accordion expand, font swap, layout settle), which read as the chat "jumping" on its own. Now it only follows while the model is actually streaming.
Send-to-bottom and the jump-to-bottom button are unaffected — they use the force path, which bypasses this gate.

2. Stop the estimate-based realign from fighting the real-DOM lock — packages/app/src/pages/session/message-timeline.tsx
virtua.scrollToIndex(align:"end") (which uses estimated offsets) was called on every streamed token, landing a hair off from the rAF loop that pins against the real DOM height — that disagreement was the frame-to-frame bounce. Now scrollToIndex only runs when the row set changes (new message/tool row) or session status flips; pure intra-row text growth is left entirely to the measured-bottom rAF lock.
Safe because timelineRowKeys is memoized with equals: sameKeys, so its reference only changes on a real row change.

3. Respect prefers-reduced-motion in JS — packages/ui/src/components/message-part.tsx
The CSS honors reduced motion everywhere, but the imperative animate() reveal in ShellSubmessage didn't — and its initial render collapses width to 0 / blurs the value. Under reduced motion it now snaps to the resting state instead of animating (and can't get stuck hidden).

Verification

• bun run typecheck — app and ui both clean.
• Unit tests — message-part, scroll-view, layout-scroll, file-tab-scroll, use-session-hash-scroll: 16 pass / 0 fail.
• ⚠️ Not covered here: the perceptual smoothness of streaming scroll needs a visual pass against a live model — not drivable headless. Changes are low-risk and reversible, but eyes on a real stream is the real acceptance test.

Deliberately deferred (happy to follow up)

• Fuller scroll consolidation — collapse to a single bottom-follow authority (CSS overflow-anchor + flex bottom-anchoring, delete the rAF workaround for virtua #301). Higher impact but needs the visual pass to tune.
• Positive motion pass — a subtle standardized enter (transform+opacity) for new message blocks; move the header spinner's width/margin animation to a compositor-only transform.

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