feat(windows): per-app screen-audio sharing (include/exclude)

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>

clients/windows/VoiceCat.Interop/Structs.cs

@@ -46,6 +46,9 @@ internal struct VcStreamDescNative
     public IntPtr DeviceId; // unused by vc_stream_start today — device selection is a
                              // separate vc_set_input_device call; always IntPtr.Zero here.
     public IntPtr Label;
+    // Mirrors vc_stream_desc::external_feed. When 1, the core skips its own WASAPI loopback
+    // and the caller feeds PCM via StreamFeedPcm (per-app capture path on Windows).
+    public int ExternalFeed;
 }
 
 [StructLayout(LayoutKind.Sequential)]