Implement semantics service facade wrapping SemanticsService

Implement the queue as a List with a bool _isProcessing flag. Use a recursive Future-based processing loop (not a Timer.periodic) to process one item, wait the configured delay, then process the next. This avoids timer drift and is easier to test. For SemanticsService.announce(), note it is a static method — wrap it in a thin protected method so subclasses can override it in tests.

The platform difference between VoiceOver (iOS) and TalkBack (Android) is largely handled by Flutter's SemanticsService.announce() — you do not need separate MethodChannel calls unless you need custom announcement modes not exposed by Flutter. AnnouncementItem: {String message, AnnouncementPriority priority, Locale? locale}. Keep the Riverpod provider as Provider (not StateProvider) since the service manages its own mutable queue.

Write unit tests using flutter_test: (1) polite announcements are queued and processed in order, (2) assertive announcement bypasses queue, (3) clearQueue removes pending items, (4) dispose cancels timers and prevents further announcements, (5) locale is passed correctly to underlying service, (6) queue processes with correct delay between items. Use a mock SemanticsService or capture calls via the abstract interface. Widget tests should verify integration with the Riverpod provider override. Minimum 85% branch coverage on the facade implementation.