feat(audio): stereo mic capture on Windows & macOS desktop clients

Both desktop mics were hard-mono: the core defaults capture_channels=1 and
neither client ever called vc_set_capture_channels (only iOS did). Add a
persisted "Stereo microphone" toggle to each client's Audio settings, applied
when the mic stream starts and live via vc_set_capture_channels + vc_audio_restart.
Expose both ABI calls in the Windows interop; the macOS wrapper already had them.

Core fix: encode_and_send_frame now folds a stereo mic frame to mono on a mono
channel - previously the channels==2 branch encoded interleaved L/R directly even
on a mono channel, feeding a mono opus_encode 2x its samples (wrong pitch/garbage).
Real stereo still only reaches the wire on a stereo channel; on a mono channel the
mic is cleanly downmixed.

Test: test_stereo_mic_mono_channel. ctest --preset dev green (28/28). Docs: voice.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.Interop/NativeMethods.cs

@@ -65,6 +65,12 @@ internal static partial class NativeMethods
     [LibraryImport(LibName, StringMarshalling = StringMarshalling.Utf8)]
     internal static partial VcResult vc_set_input_device(nint c, uint streamId, string? deviceId);
 
+    [LibraryImport(LibName)]
+    internal static partial VcResult vc_set_capture_channels(nint c, uint streamId, uint channels);
+
+    [LibraryImport(LibName)]
+    internal static partial VcResult vc_audio_restart(nint c);
+
     [LibraryImport(LibName)]
     internal static partial VcResult vc_set_input_mode(nint c, VcInputMode mode);
 

clients/windows/VoiceCat.Interop/VoiceCatClient.cs

@@ -212,6 +212,19 @@ public sealed class VoiceCatClient : IDisposable
     public VcResult SetInputDevice(uint streamId, string? deviceId) =>
         NativeMethods.vc_set_input_device(_handle.DangerousGetHandle(), streamId, deviceId);
 
+    /// <summary>Sets the mic capture channel count (1 = mono, 2 = stereo) for the given stream.
+    /// Applied when the capture device next (re)starts — call before Join Voice, or pair with an
+    /// audio restart to take effect live. Real stereo only reaches the wire on a stereo channel;
+    /// the core folds a stereo mic to mono on a mono channel.</summary>
+    public VcResult SetCaptureChannels(uint streamId, uint channels) =>
+        NativeMethods.vc_set_capture_channels(_handle.DangerousGetHandle(), streamId, channels);
+
+    /// <summary>Uninitializes and re-initializes the capture and playback devices on a running
+    /// engine, applying pending changes (e.g. capture channel count) that only take effect on a
+    /// device restart. No-op if audio isn't running.</summary>
+    public VcResult AudioRestart() =>
+        NativeMethods.vc_audio_restart(_handle.DangerousGetHandle());
+
     public VcResult SetInputMode(VcInputMode mode) =>
         NativeMethods.vc_set_input_mode(_handle.DangerousGetHandle(), mode);