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.App/Forms/MainForm.Designer.cs

@@ -36,6 +36,7 @@ partial class MainForm
     private FlowLayoutPanel flpVoiceTop = null!;
     private FlowLayoutPanel flpVoiceBottom = null!;
     private Button btnMicToggle = null!;
+    private Button btnScreenShareToggle = null!;
     private CheckBox chkMute = null!;
     private CheckBox chkDeafen = null!;
     private RadioButton radioVad = null!;
@@ -81,6 +82,7 @@ partial class MainForm
         flpVoiceTop = new FlowLayoutPanel();
         flpVoiceBottom = new FlowLayoutPanel();
         btnMicToggle = new Button();
+        btnScreenShareToggle = new Button();
         chkMute = new CheckBox();
         chkDeafen = new CheckBox();
         radioVad = new RadioButton();
@@ -227,6 +229,20 @@ partial class MainForm
         btnMicToggle.Margin = new Padding(0, 2, 6, 0);
         btnMicToggle.TabIndex = 0;
 
+        // Screen-audio share — independent of mic voice (can share without joining voice and
+        // vice versa). The core's WASAPI loopback path (VOICECAT_HAS_LOOPBACK, always on for
+        // the windows-client preset) captures the default render endpoint; see docs/voice.md
+        // §9. Whole-device loopback inherently re-captures this app's own incoming voice mix —
+        // an accepted self-echo characteristic, not a bug.
+        btnScreenShareToggle.Text = "Share Screen &Audio";
+        btnScreenShareToggle.AccessibleName = "Share screen audio";
+        btnScreenShareToggle.AccessibleDescription =
+            "Start or stop sharing your computer's audio (desktop/system audio) with the channel. " +
+            "Independent of the microphone. Captures everything playing through your default speakers.";
+        btnScreenShareToggle.AutoSize = true;
+        btnScreenShareToggle.Margin = new Padding(0, 2, 12, 0);
+        btnScreenShareToggle.TabIndex = 10;
+
         chkMute.Text = "&Mute mic";
         chkMute.AutoSize = true;
         chkMute.Enabled = false;
@@ -276,6 +292,7 @@ partial class MainForm
         flpVoiceTop.AutoSize = false;
         flpVoiceTop.Padding = new Padding(4, 2, 4, 0);
         flpVoiceTop.Controls.Add(btnMicToggle);
+        flpVoiceTop.Controls.Add(btnScreenShareToggle);
         flpVoiceTop.Controls.Add(chkMute);
         flpVoiceTop.Controls.Add(chkDeafen);
         flpVoiceTop.Controls.Add(lblMode);

clients/windows/VoiceCat.App/Forms/MainForm.cs

@@ -23,6 +23,7 @@ public partial class MainForm : Form
 
     // Voice state
     private uint _micStreamId; // 0 = not started
+    private uint _screenStreamId; // 0 = not sharing screen audio
     private Keys _pttKey = Keys.F8;
     private bool _serverMuted;
     private bool _serverDeafened;
@@ -60,6 +61,7 @@ public partial class MainForm : Form
 
         // Voice controls
         btnMicToggle.Click += BtnMicToggle_Click;
+        btnScreenShareToggle.Click += BtnScreenShareToggle_Click;
         chkMute.CheckedChanged += (_, _) => ApplySelfMute();
         chkDeafen.CheckedChanged += (_, _) => ApplySelfMute();
         radioVad.CheckedChanged += RadioVad_CheckedChanged;
@@ -404,9 +406,11 @@ public partial class MainForm : Form
         _talkingUsers.Clear();
         _currentChannelId = 0;
         _micStreamId = 0;
+        _screenStreamId = 0;
         txtCompose.Enabled = false;
         btnSend.Enabled = false;
         btnMicToggle.Enabled = false;
+        btnScreenShareToggle.Enabled = false;
     }
 
     // ── Level meter ───────────────────────────────────────────────────────────
@@ -604,6 +608,35 @@ public partial class MainForm : Form
     private void ApplySelfMute() =>
         _client.SetSelfMute(chkMute.Checked, chkDeafen.Checked);
 
+    // Screen-audio share — independent of mic voice. The core bypasses VAD/PTT/self-mute/
+    // server-mute for non-MIC kinds (client.cpp on_capture_frame), so no input-mode or mute
+    // state applies here. WASAPI loopback captures the default render endpoint (whole-device,
+    // not process-specific — docs/voice.md §9).
+    private void BtnScreenShareToggle_Click(object? sender, EventArgs e)
+    {
+        if (_screenStreamId == 0)
+        {
+            var (result, streamId) = _client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
+            if (result == VcResult.Ok)
+            {
+                _screenStreamId = streamId;
+                btnScreenShareToggle.Text = "Stop Screen &Audio";
+                AddActivity("Started sharing screen audio");
+            }
+            else
+            {
+                AddActivity($"Failed to start screen audio: {result}");
+            }
+        }
+        else
+        {
+            _client.StopStream(_screenStreamId);
+            _screenStreamId = 0;
+            btnScreenShareToggle.Text = "Share Screen &Audio";
+            AddActivity("Stopped sharing screen audio");
+        }
+    }
+
     private void RadioVad_CheckedChanged(object? sender, EventArgs e)
     {
         if (!radioVad.Checked) return;
@@ -916,6 +949,11 @@ public partial class MainForm : Form
         _pumpTimer.Stop();
         _client.LevelChanged -= OnLevelChanged;
         _client.EventReceived -= OnEvent;
+        // Stop any active local streams before tearing down — the core stops loopback in
+        // stream_stop/destroy, but explicit stops ensure clean StreamStop protocol messages
+        // go out before Disconnect closes the control channel.
+        if (_screenStreamId != 0) _client.StopStream(_screenStreamId);
+        if (_micStreamId != 0) _client.StopStream(_micStreamId);
         _client.Disconnect();
         _client.Dispose();
         base.OnFormClosed(e);

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();
+    }
 }