Apply same architecture as FlacCacheSink to prevent file truncation
when external readers access files during encoding.
Changes:
1. Add pump_track_segments_from_channel() for parallel pump tasks
2. Refactor process() to use dispatcher + tokio::select! pattern
3. Create .complete marker after flush/wait to signal file is ready
4. Allow multiple tracks to encode in parallel (pump continues in background)
This ensures FlacFileSink is cache progressive compliant, meaning external
code can safely read output files while they're being written without
risk of truncation.
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