- Add Node::logic_mut() method to allow post-construction configuration
of node logic before run() is called
- Fix FlacCacheSink to properly transfer playlist_handle_pending to
the inner logic using logic_mut()
- Resolves FIXME at flac_cache_sink.rs:656 about missing logic_mut()
This enables the playlist registration mechanism to work correctly:
1. User calls register_playlist() on FlacCacheSink
2. Handle is stored in playlist_handle_pending
3. During run(), handle is transferred to FlacCacheSinkLogic
4. Tracks are automatically added to playlist after caching
