Previous fix consumed the TrackBoundary with drain_until_track_boundary(),
preventing the next track from being processed correctly. This caused
audio to stop after the first cached track.
Solution: Use a pump_closed flag instead of draining. When the pump
closes early (cache hit), set the flag and ignore subsequent chunks
until TrackBoundary. The TrackBoundary is then handled normally by
the existing code, allowing proper continuation to the next track.
This preserves the block structure and allows all tracks in a block
to be processed correctly, whether cached or not.
413047cce5