Fix "PlaylistManager not initialized" error in play_and_cache example.
The error occurred because pmoplaylist's WriteHandle calls
pmoupnp::get_audio_cache() to validate cache pks, but the global
cache registry wasn't initialized.
Changes:
- Add register_audio_cache() and register_cover_cache() functions
- Export them from pmoupnp lib.rs
- Call them in play_and_cache example after creating caches
This mirrors how UpnpServer initializes the cache registry.
