Add bounded microphone capture requests and MediaStreamTrack lifecycle #775

Parent: #770
Prerequisite: #773 (merged by #774)
Native providers: SceneTech/AppScene#289

Implement the bounded microphone request and MediaStreamTrack lifecycle: parse the exact audio constraint subset used by unchanged Code OSS, enqueue an origin-scoped typed host open request, validate the host completion and negotiated format, create one branded live audio track, accept bounded timestamped float PCM/events through a size-versioned engine API, and stop/release on track stop, cancellation, navigation, realm retirement, device loss, or engine shutdown. No synthetic successful stream and no realtime JS/allocation/lock/I/O path.

Author deterministic injected-provider tests for constraints, denial, default/exact device, stale generation, ordering, mute/unmute/ended, overflow, navigation and teardown. Execution is deferred under the current throughput directive.