feat(windows): wire screen-audio sharing into the WinForms client

The core already supported SCREEN_AUDIO capture on Windows (post-M3 WASAPI
loopback via VOICECAT_HAS_LOOPBACK) and the C# Interop layer was complete
(VcStreamKind.ScreenAudio, StartStream/StopStream/SetRemoteStream). Only the
UI was missing -- no core, proto, or C ABI changes needed.

Adds a 'Share Screen Audio' toggle to the voice panel, independent of mic
voice (can share without joining voice). Disconnect/teardown now stops the
screen stream cleanly. New smoke test exercises the full StartStream ->
StreamStarted -> StopStream -> StreamStopped path through P/Invoke.

clients/windows/VoiceCat.Interop.Tests/VoiceCatClientSmokeTests.cs

@@ -246,4 +246,51 @@ public sealed class VoiceCatClientSmokeTests : IDisposable
 
         client.Disconnect();
     }
+
+    /// <summary>
+    /// Screen-audio (SCREEN_AUDIO) stream start/stop through the P/Invoke layer. The core's
+    /// WASAPI loopback path (VOICECAT_HAS_LOOPBACK) captures the default render endpoint; the
+    /// StreamAnnounce succeeds regardless of whether the loopback device actually initializes
+    /// on a headless box, so this test passes in CI while still exercising the full
+    /// StartStream -> StreamStarted -> StopStream -> StreamStopped path through P/Invoke.
+    /// See docs/voice.md §9 and MainForm's BtnScreenShareToggle_Click.
+    /// </summary>
+    [Fact]
+    public void ScreenAudioStream_Starts_And_Stops()
+    {
+        var events = new List<VoiceCatEvent>();
+        using var client = new VoiceCatClient("vc-csharp-screen", "0.1", VcLogLevel.Off,
+            tofuStorePath: Path.Combine(_tempDir, "tofu_pins_screen.txt"));
+        client.EventReceived += events.Add;
+
+        Assert.Equal(VcResult.Ok, client.Connect("127.0.0.1", _port));
+        Assert.Equal(VcResult.Ok, client.AuthenticateGuest("CSharpScreen"));
+
+        Assert.True(PumpUntil(client, () => events.Any(e => e.Type == VcEventType.ServerIdentity), 5000));
+        Assert.Equal(VcResult.Ok, client.ConfirmServerIdentity(accept: true));
+        Assert.True(PumpUntil(client, () => events.Any(e => e.Type == VcEventType.AuthResult), 5000));
+        Assert.Equal(VcResult.Ok, events.First(e => e.Type == VcEventType.AuthResult).Result);
+        Assert.True(PumpUntil(client, () => events.Any(e => e.Type == VcEventType.ChannelList), 3000));
+
+        // Give the async UDP binding handshake a moment to land before announcing a stream
+        // (mirrors vccli's 500ms sleep after auth).
+        Thread.Sleep(500);
+
+        var (startResult, streamId) = client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
+        Assert.Equal(VcResult.Ok, startResult);
+        Assert.True(streamId != 0, "StreamId should be non-zero on success");
+
+        // The core emits VC_EVENT_STREAM_STARTED for the local client too (client.cpp
+        // handle_stream_announce_result), so we see our own screen-audio stream start.
+        Assert.True(PumpUntil(client,
+            () => events.Any(e => e.Type == VcEventType.StreamStarted && e.StreamId == streamId), 5000),
+            "did not receive VC_EVENT_STREAM_STARTED for screen-audio stream");
+
+        Assert.Equal(VcResult.Ok, client.StopStream(streamId));
+        Assert.True(PumpUntil(client,
+            () => events.Any(e => e.Type == VcEventType.StreamStopped && e.StreamId == streamId), 5000),
+            "did not receive VC_EVENT_STREAM_STOPPED for screen-audio stream");
+
+        client.Disconnect();
+    }
 }