Snap outer timeline growth while a thread is active

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The top-level timeline retained a 180 ms height transition while its thread was active. Bottom anchoring followed the intermediate heights, moving existing rows repeatedly for a single content update. Reproduction and root-cause report.

What changed

Disable outer height growth animation using the timeline's existing active state. Preserve idle easing and existing resize/revision behavior. Three files; 39 changed text lines (37 additions, 2 deletions).

How you verified

• Added a real-timeline regression test before the production fix. It failed on trusted main in two clean checkouts: expected height 0ms, received height 180ms.
• pnpm exec turbo run test --filter=@bb/app -- ThreadTimelineRows height-transition bottom-anchored-scroll-body: 110 tests passed across 15 files after the fix.
• pnpm exec turbo run typecheck --filter=@bb/app: passed.
• Headless Chrome with real height/scroll components and synthetic rows: one 30 px update produced intermediate scroll positions on main; the fixed probe snapped directly to the final height. Packaged desktop/provider integration was not exercised.
• git diff --check: passed. No binary changes.

Fixes #3759

AGENT GENERATED