fix(replay): Release MediaMuxer when no frames are encoded (#5583)

* fix(replay): Release MediaMuxer when no frames are encoded

The MediaMuxer is created when the video encoder is constructed, but its
release() was reachable only on the happy path. Two cases leaked it:

- createVideoOf returned early when frameCount was 0 without releasing the
  encoder.
- SimpleMp4FrameMuxer.release() called muxer.stop() before muxer.release().
  stop() throws if the muxer was never started (no frame ever muxed), so
  release() was skipped.

This surfaced as a CloseGuard "resource was acquired but never released"
warning. Guard stop() behind the started flag so release() is always
reached, and release the encoder on the no-frames return path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* changelog

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>