Audio: add independent payload lifecycle

[mohammadaaftabv]

Summary

• add generic admission-controlled PayloadRef storage, bounded prefetch, release, and dropped-task cleanup
• materialize audio once with segment-aware FFmpeg I/O and let FastConformer consume waveform payloads
• preserve structural batch envelopes through a payload-neutral items / with_items contract
• wrap consumers that do not directly implement payload resolution, keeping this feature independent of global bucketing

Independence

This PR is based on main and does not contain Qwen-Omni, performance instrumentation, BatchPolicy, dispatch batches, or global planning. It can merge before or after the global-bucketing PR.

Test plan

• Ruff check and format validation
• Payload lifecycle, ref, prefetch, dropped-task, and audio-reader tests
• FastConformer waveform-consumption proof
• Structural-envelope materialize → consume → release proof
• Merge-order composition validation with the independent global-bucketing branch

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