Changes:
1. pmocache/cache_trait.rs - Fixed is_valid_pk() logic:
- Accept files WITH completion markers (complete downloads)
- Accept files WITHOUT markers but recent (< 60s) (downloads in progress)
- Reject files WITHOUT markers and old (>= 60s) (failed downloads)
This preserves progressive caching: files are valid as soon as prebuffer
completes, without waiting for completion marker.
2. pmoupnp/cache_registry.rs - Added compatibility layer:
- Re-exports get_audio_cache/get_cover_cache from singletons
- Provides build_audio_url/build_cover_url for pmosource
- Uses PMO_SERVER_URL env var for base URL
3. pmoupnp/lib.rs - Added cache_registry module to public API
This fixes "Cache entry not found" errors while maintaining progressive
caching functionality for play_and_cache example.
ff859372cf