Summary

• add in-memory cache to RuntimeStateManager so the notification hot path avoids redundant disk reads
• load_state and load_section serve from cache after the first disk read
• save_state writes through to disk and updates the cache atomically
• skip os.fsync on Windows where os.replace is atomic enough
• fix legacy-section detection to check whether the unified state file actually exists rather than checking for a populated dict (which could be a default)

Effect

• before: every notification check did a full disk read + possibly a disk write
• after: only the first read hits disk; subsequent reads and all saves are cache-first

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