fix(start): a second instance can no longer kill the app on Go Live

Reported 2026-11-06: a Release instance crashed when clicking Go Live in test mode, while a Debug
instance was recording its window. The Windows event log gave the cause:

    System.IO.IOException: All pipe instances are busy.
    at System.IO.Pipes.NamedPipeServerStream.Create(...)
    at ytLive.Services.Audio.NamedPipeAudioWriter.Start(...)
    at ytLive.Services.Audio.AudioMixer.StartLive(...)
    at MainViewModel.StartStreamingSession(...)
    at MainViewModel.BeginTestStream(...)
    at MainViewModel.StartFromPrimaryButton(...)
    at System.Windows.Controls.Button.OnClick()

Two defects, both fixed:

1. **A button click could end the process.** `StartLive` throws when the pipe name is taken, and nothing
   on the go-live path caught it. `TryStartAudioPipe` now catches, logs, toasts ("Another copy of
   LlamaCasty is already using the audio pipe") and rolls the session back. Recording-only takes the same
   path.
2. **The pipe name was a shared constant**, so a second instance could never go live on the same machine.
   `InstanceProfile.AudioPipeName` appends the dev instance id (DEBUG only); Release keeps the plain name
   so the normal single-instance case is unchanged. The encoder reads the same derived name, so mixer and
   ffmpeg cannot disagree.

Tests: `AudioPipeInstanceTests` — a second server on one name throws (the collision is real), dev names
derive per instance, and a writer on a taken name throws rather than silently reusing the pipe.
Debug and Release both build clean.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -66,12 +66,34 @@ public partial class MainViewModel : ViewModelBase
         _liveElapsed = TimeSpan.Zero;
         _recordLength = TimeSpan.Zero;
         ResetHealth(StreamStatus.Offline);
-        _audioMixer.StartLive(EncoderOptions.DefaultAudioPipeName);
+        if (!TryStartAudioPipe()) return;
         IsRecording = true;
         LiveScene = StagedScene;
         await _framePump.StartAsync();
     }
 
+    /// <summary>Starts the live audio pipe, or explains why it cannot and rolls the session back.
+    /// <para>This used to throw straight out of the Go Live button: the pipe server allows one instance,
+    /// so with a second LlamaCasty running on the same machine the click killed the app
+    /// ("All pipe instances are busy", 2026-11-06). A click must never be able to end the process.</para>
+    /// </summary>
+    private bool TryStartAudioPipe()
+    {
+        try
+        {
+            _audioMixer.StartLive(InstanceProfile.AudioPipeName(EncoderOptions.DefaultAudioPipeName));
+            return true;
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write(ex, "Audio pipe could not be started — session aborted");
+            _notifications.Error("Couldn't start the audio",
+                "Another copy of LlamaCasty is already using the audio pipe. Close it, then try again.");
+            StopStream();
+            return false;
+        }
+    }
+
     private void BeginGoLive(bool alsoRecord)
     {
         var dialog = new GoLiveViewModel(() => SignInAsync(), _youtubeAuth.CurrentChannel)
@@ -124,7 +146,7 @@ public partial class MainViewModel : ViewModelBase
         // persisted, and reset when the session ends.
         _sessionStartedAtUtc = DateTime.UtcNow;
         ResetHealth(StreamStatus.Streaming);
-        _audioMixer.StartLive(EncoderOptions.DefaultAudioPipeName);
+        if (!TryStartAudioPipe()) return;
         _liveElapsed = TimeSpan.Zero;
         _recordLength = TimeSpan.Zero;
 
@@ -471,6 +493,8 @@ public partial class MainViewModel : ViewModelBase
             InputHeight = compositor.OutputHeight,
             Fps = quality.Fps,
             BitrateKbps = (int)Math.Round(quality.Bitrate * 1000),
+            // Same derivation as the mixer's StartLive call: a dev instance serves its own pipe.
+            AudioPipeName = InstanceProfile.AudioPipeName(EncoderOptions.DefaultAudioPipeName),
         };
     }