feat: support aria-busy state announcement

[AdamMagued]

Summary

Implements support for the aria-busy state per the W3C WAI-ARIA 1.2 specification, addressing issue #36.

Changes

1. Accessibility Tree and Node Data:

   • Detects aria-busy="true" on elements and sets busy: boolean on AccessibilityNode.
   • Propagates busy across root and child tree nodes.
   • Maps attribute labels (busy for true, not busy for false).

2. Focused Element Announcements:

   • Implemented #announceFocusedBusy in Virtual.ts to observe mutations on active/focused elements.
   • Announces "busy" when aria-busy transitions to "true" and "not busy" when set to "false".
   • Avoids duplicate announcements when the focused element is also an active live region.

3. Live Region Processing:

   • Tracks busy: boolean state across ancestor hierarchies in getLiveRegionAttributes.
   • Announces ${live}: busy when a live region becomes busy.
   • Suppresses intermediate DOM updates while a live region or its ancestors remain busy.
   • Automatically announces updated region content when aria-busy is cleared or set to "false".

4. Tests and Coverage:

   • Added comprehensive integration tests in test/int/ariaBusy.int.test.ts covering static navigation, accessibility tree attributes, dynamic focused changes, live regions, nested busy ancestry, batch mutations, and edge cases.
   • All 58 test suites (358 tests) pass with 100% test pass rate and satisfy all strict coverage thresholds (statements >= 98%, branches >= 95%, functions 100%, lines >= 98%).

Closes #36