Add _dead set to _IntrospectionCache to track paths that timeout.
Failed introspections are skipped on future lookups, reducing
timeout blocks from 300-500ms to near-zero.
- Timeouts are expected for services without introspection support
- _dbus_get_pid and _dbus_introspect now log at debug level
- Cache already stores empty results; invalidation on service appear
causes retries which is correct behavior
- wallpapers_dir: read from config, fallback ~/Pictures/Wallpapers
- osd: skip show_audio_osd/show_brightness_osd when disabled
- window_switcher: skip show_switcher when disabled
- global_menu_cache_enabled: bypass GTK class cache when false
- global_menu_method: control discovery strategies (auto/hyprland/dbus)
All keys added to constants.py defaults, settings UI, and config.toml