fix(audio): stereo screen-audio loopback capture on Windows

start_loopback_capture hardcoded channels=1, forcing miniaudio to downmix the system's stereo mix to mono before the encoder saw it -- on_capture_frame then upmixed L=R to produce fake stereo. Now the loopback device opens in the channel's mode (stereo when the channel is stereo), CaptureCallback carries an explicit channels param so the encoder gets real interleaved L/R, and a mono fallback covers unusual render endpoints. New test_loopback_stereo_capture asserts L!=R end-to-end; 18/18 ctest green.

core/src/core/client.h

@@ -272,7 +272,7 @@ struct vc_client {
     void run_udp_recv();
     // capture_cb passed to audio_engine_.start(): encode + seal + send one frame for the
     // given local stream `kind` (M3: multiple concurrent local streams are possible).
-    void on_capture_frame(int kind, const int16_t* pcm, int samples);
+    void on_capture_frame(int kind, const int16_t* pcm, int samples, int channels);
     // Inspect a User proto's streams and wire up any new remote ssrc into audio_engine_,
     // emitting VC_EVENT_STREAM_STARTED/STOPPED as streams appear/disappear.
     void sync_remote_streams(const voicecat::v1::User& user);