bug(react): viewport tracking rescans and retains removed descendants

[vamgan]

Summary

React viewport tracking has mutation-scaling and stale-state problems in dynamic/virtualized UIs.

Evidence

packages/react/src/useAskableViewport.ts:

1. Relevant additions call observeAll(), rescanning every annotation. Sequential inserts become approximately O(existing annotations × mutations).
2. When an annotated parent is removed, cleanup processes the parent or descendants, not both. Annotated descendants can remain in the visible map.
3. Changing only scope updates a ref but does not flush output, leaving stale visibleItems until another observer callback.

Proposed fix

• Observe only each added node plus annotated descendants.
• On removal, process both an annotated root and every annotated descendant.
• Flush immediately on scope changes or derive filtered output from unfiltered state.
• Consider sharing core viewport machinery to avoid duplicate implementations.

Acceptance criteria

• Removing an annotated parent clears all annotated descendants
• Scope changes update visible output immediately
• Adding one node does not rescan the full document
• Sequential-insertion benchmark counts selector scans and observer registrations
• Virtualized-list regression coverage is added