chore(windows): add audio device-state watcher for headphone dropout diagnosis (#724)

[abduznik]

Draft PR for #724 — this is diagnostic instrumentation only, not a fix yet.

Per the discussion on the issue, before committing to a keep-alive-stream fix we need real evidence on which of three mechanisms is causing headphones to drop mid-recording: USB selective suspend, WASAPI render-endpoint idle, or the headset's own firmware auto-off timer. A silent keep-alive stream only helps with the first two.

This adds WasapiDeviceWatcher, an IMMNotificationClient that logs render/capture endpoint state transitions as structured JSON events for the duration of a recording. It changes no recording behavior — opt-in only, via OPENSCREEN_WGC_LOG_AUDIO_DEVICE_EVENTS=1.

Opening this as a draft mainly to get Windows CI to compile-check it (my local VS Build Tools install is currently broken and I can't build locally yet). Will report back on the issue once I've captured real device-state events during a reproduction on my hardware.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

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