fix(18): stop ends everything — pills clear, pump/prep failures roll the session back

Creator ruling 2026-09-01 (stuck REC pill after the failed first recording):
- StopStream clears RecordPillOn + OnAirPillOn — intent resets with reality.
- OnFramePumpFailed: dispatcher-marshalled FULL rollback via StopStream (was:
  toast + Error-status only when live — record-only sessions zombied with
  IsRecording=true over a dead encoder; the 20:34 attempt proved it). Toast copy
  says 'Recording stopped' vs 'stream pipeline stopped'.
- Same zombie class in the three go-live prep failure branches: StreamStatus.Error
  limbo replaced by StopStream() (audio loop + recording + pills unwind; a created
  broadcast still gets the close-out).
- AudioMixer.StopLive made explicitly idempotent (Stop() twice, rollback paths that
  never reached StartLive).
- Seams: OnFramePumpFailed + IsRecording setter internal (InternalsVisibleTo; test
  pattern mirrors LayoutPathOverride).
ONE integration test: SessionTeardownTests (real window + temp DB: pump death with
lit pill -> no zombie, no End button, Offline). AudioPipelineTests confirmed to hang
STANDALONE (pre-existing, the declared-known audio class) — ai.md test-count
paragraph corrected to stop claiming a suite total that cannot currently be measured.

ViewModels/MainViewModel.Streaming.cs

@@ -107,11 +107,13 @@ public partial class MainViewModel : ViewModelBase
     public bool CanToggleOnAir => IsConnected;
 
     /// <summary>Whether the session is actively recording — the REC status light
-    /// turns green only when this is true (the pill is intent, this is reality).</summary>
+    /// turns green only when this is true (the pill is intent, this is reality).
+    /// Internal setter is a test seam (same pattern as LayoutPathOverride) so the
+    /// failure-rollback path can be exercised without running the real pipeline.</summary>
     public bool IsRecording
     {
         get => _isRecording;
-        private set
+        internal set
         {
             if (!SetProperty(ref _isRecording, value)) return;
             OnPropertyChanged(nameof(RecDotBrush));