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: urgent alert notifications (AlertNotificationSystem) and current conditions updates (MainWindow) gated on meaningful changes
• Tests for ring buffer, dispatcher, threshold gating, and main window emission

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 and can optionally mirror toast notifications.

Follow-up (Phase 2)

• Event center UI for browsing channels/events and read-state controls
• Hourly/daily/discussion channel producers
• Cursor/pagination hooks

Validation

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

Rebase note (2026-03-17)

Rebased onto latest dev. One conflict in main_window.py — kept both the new main_window_notification_events import (from dev) and the WeatherEvent/should_emit_current_conditions_event imports (from this PR). All 9 feature tests pass.