When a file was already in cache, FlacCacheSink would drain all
remaining segments (which can take 13+ seconds - the full track
duration) BEFORE adding the track to the playlist. This caused
a long delay before playback could start.
The fix reorders operations to:
1. Copy metadata to cache (fast)
2. Add pk to playlist IMMEDIATELY (fast)
3. Drain remaining segments (slow, but playback already started)
This ensures the playlist receives tracks immediately, allowing
playback to start without waiting for segment drainage to complete.
Fixes the 13-second delay when playing already-cached files.
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