Problem: When TrackBoundary arrived, the pump was awaited before continuing,
causing file truncation when pcm_tx was dropped while data was still buffering.
Solution: Allow multiple pump tasks to run in parallel:
- Create dedicated channel (track_tx/track_rx) for each track's pump
- Main loop reads from rx and dispatches segments to current pump via track_tx
- When TrackBoundary arrives: drop track_tx (signals pump to finish) and immediately start new pump
- Old pump continues writing in background until all data is flushed
This prevents truncation in progressive cache scenario (radio streaming).
Changes in flac_cache_sink.rs:
- Replace pump_track_segments_owned() with pump_track_segments_from_channel()
- Remove rx ownership passing - each pump gets its own channel
- Dispatcher loop reads rx and forwards to active pump
- No await on pump completion - let it finish in background
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