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.

HANDOFF.md

@@ -37,4 +37,7 @@
 - Nothing in the suite drives the real cursor any more; the drag arithmetic is pure (`DragReorderMathTests`).
 - Integration tests constructing `MainWindow` must set a temporary `LayoutPathOverride` and clear it in `finally`.
 - Media sessions are keyed by **path + look** (`MainViewModel.MediaSessionKey`); never acquire by raw path.
+- **The audio pipe is one instance per machine.** `NamedPipeAudioWriter` allows a single server, so two
+  LlamaCasty processes cannot both go live. A DEBUG instance appends its id (`InstanceProfile.AudioPipeName`),
+  Release keeps the plain name; the Go-Live path catches the failure and reports instead of dying.
 - Bundled FFmpeg stays outside the single-file exe (`ExcludeFromSingleFile="true"`).