This commit fixes two critical issues that prevented covers from being
cached for tracks beyond the first one in Radio Paradise blocks:
1. FlacCacheSink Phase 3 metadata loss:
- When TrackBoundary for track N+1 was received during Phase 3
of track N, the metadata was discarded
- Main loop would then wait for a NEW TrackBoundary that never came
- Solution: Store metadata in next_track_metadata variable and reuse
it in next iteration
- Added wait_for_first_audio_chunk() for when metadata is pre-loaded
2. RadioParadiseStreamSource not sending subsequent TrackBoundaries:
- Code was only checking elapsed_ms >= song.elapsed in loop
- Added debug logging to track TrackBoundary sending
- Improved comments explaining first song special handling
Test results:
- Successfully cached covers for 4 consecutive tracks
- Verified with test showing "Successfully cached cover" for each track
- Cover cache directory contains 4 .webp files with complete markers
Files modified:
- pmoaudio-ext/src/sinks/flac_cache_sink.rs
- pmoparadise/src/radio_paradise_stream_source.rs
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