Summary

• migrate alert runtime state into the unified runtime store with per-section legacy fallback
• migrate notification event state into the unified runtime store while preserving AFD and severe-risk continuity
• keep portable/settings copy flows excluding runtime state and add migration-focused regression coverage

Testing

• pytest tests/test_runtime_state_manager.py tests/test_alert_manager.py tests/test_notification_event_manager.py tests/test_split_notification_timers.py tests/test_settings_dialog_portable_copy.py -q
• ruff check src/accessiweather/runtime_state.py src/accessiweather/alert_manager.py src/accessiweather/notifications/notification_event_manager.py src/accessiweather/ui/main_window_notification_events.py src/accessiweather/app_initialization.py tests/test_runtime_state_manager.py tests/test_alert_manager.py tests/test_notification_event_manager.py tests/test_split_notification_timers.py

Closes #467