Feed native microphone PCM through MediaStreamAudioSourceNode #776

Parent: #770
Prerequisites: #773 and the microphone capture-track child

Implement AudioContext.createMediaStreamSource() / MediaStreamAudioSourceNode for live native microphone tracks. Feed bounded PCM into the existing fixed-quantum audio graph with explicit sample-rate/channel conversion, silence for disabled/muted tracks, analyser visibility, no output duplication, and deterministic stop/context-close/navigation teardown. Keep the native producer callback free of JavaScript, allocation, locks, logging and I/O.

Author injected PCM tests for mono 16 kHz conversion, analyser values, discontinuity/drop reporting, mute/unmute, switching, graph connect/disconnect, latency, queue bounds and 100-cycle resource plateaus. Execution is deferred under the current throughput directive.