MoqSrc/MoqSink: refactor to async session tasks with channel-based pad forwarding (#36)

* moqsink: refactor to async session controller with dynamic pad forwarding

* Refactor MoqSrc into an async element with channel-driven pad forwarding and H.264/H.265/AV1 caps support.

* Change mutability due to warning

* cargo fmt

* Address PR review feedback

- Remove worker_threads(4) from both runtimes (Quinn doesn't scale past 1)
- Switch sink to unbounded channel
- Make sink run_session return Result<()> for centralized error handling
- Extract spawn body into named run_track_pump function in source
- Remove unnecessary forwarder test
- Minor cleanups: remove #![allow(dead_code)], simplify destructure, collapse _session binding

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Luke Curley <kixelated@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people