Adds "only selected apps" and "all apps except selected" screen-audio
modes to the Windows client, alongside the existing entire-desktop path.
Per-app capture uses WASAPI process loopback (AUDCLNT_ACTIVATIONTYPE_
PROCESS_LOOPBACK) via ProcessLoopbackCapture, mixed by ProcessAudioMixer
and fed to the core through vc_stream_feed_pcm (external_feed=1 so the
core skips its own loopback device).
Init must pass AUDCLNT_STREAMFLAGS_LOOPBACK | EVENTCALLBACK |
AUTOCONVERTPCM; the LOOPBACK flag is what makes the virtual endpoint
deliver rendered audio (without it every buffer is flagged SILENT) and
AUTOCONVERTPCM resamples the app's native format to 48k s16.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
