feat(events): add phase-1 channelized weather event pipeline

[Orinks]

Summary

Implements Phase 1 infrastructure for a channelized weather event pipeline to support dual-mode weather presentation (visible UI + screen-reader/invisible announcements) while preserving existing full views.

Scope

• Add WeatherEvent domain model and in-memory ring buffer store (default size 300)
• Add WeatherEventDispatcher.dispatch_event(event, *, announce=True, mirror_toast=True)
• Wire app-level retrieval hooks:
  • get_latest_event(channel=None)
  • get_unread_summary()
• Integrate two producers only:
  • Urgent alert notifications path (AlertNotificationSystem)
  • Current conditions update path (MainWindow) gated on meaningful changes
• Add tests for:
  • Store ring buffer behavior + unread counts
  • Dispatcher write/announce/toast behavior
  • Current conditions threshold gating

Behavior notes

• Existing current conditions/forecast UI remains unchanged.
• Existing toast behavior for alert notifications is preserved.
• Event dispatcher mirrors to accessibility output via existing screen reader path (Prism/Prismatoid wrapper) and can optionally mirror toast notifications.

Follow-up (Phase 2)

• Add event center UI for browsing channels/events and read-state controls
• Add hourly/daily/discussion channel producers
• Expand cursor/pagination hooks for richer visible/invisible workflows

Validation

• ruff check on touched files
• pytest -q tests/test_weather_event_pipeline.py tests/test_alert_lifecycle.py