Fix flaky timeline scroll-to-bottom restores

[ymichael]

Summary:

• Cancel a pending saved-row restore when an explicit scroll-to-bottom request happens, so a later ResizeObserver settle pass cannot pull the timeline back up.
• Add a latched user-detached-from-bottom state so streaming/layout drift can save bottom intent, while actual manual scrolls still save the visible row.
• Flush the final scroll anchor from the effect-captured scroll element during unmount, since refs can already be nulled.
• Add regression tests for stale saved-row restore undoing scroll-to-bottom, streaming drift on unmount, and fast unmount after manual wheel scroll.

Validation:

• pnpm exec turbo run test --filter=@bb/app -- src/components/ui/bottom-anchored-scroll-body.scroll-preservation.test.tsx
• pnpm exec turbo run typecheck --filter=@bb/app

[ymichael]

Temporarily reverted the branch changes in b13269a so the PR branch has no net diff from main while testing scroll behavior. The original fix commits remain in history and can be restored by reverting this revert commit.

[ymichael]

Updated the branch with a smaller candidate after baseline testing: the PR now keeps main's scroll-preservation capture semantics and only cancels a pending saved-row restore on explicit scroll-to-bottom. This is ready for another manual test pass.

[ymichael]

Added the latched-detach candidate in a6c5812. This version should preserve bottom when streaming/layout drift leaves the scrollport temporarily off-bottom, but still preserve a row after actual user scroll, including fast switch-away before a scroll event. Ready for the next manual test pass.