Fix "PlaylistManager not initialized" error without creating circular
dependency between pmoparadise and pmoupnp.
Changes:
- Add AUDIO_CACHE static to pmoplaylist/manager.rs
- Add register_audio_cache() function to register cache
- Export register_audio_cache from pmoplaylist lib.rs
- Update audio_cache() to check local registry first, then pmoupnp
- Update play_and_cache example to use pmoplaylist::register_audio_cache
- Remove pmoupnp::register_*_cache functions (not needed)
The example now calls pmoplaylist::register_audio_cache() to make
the cache available for pk validation in WriteHandle::push().
