feat(alerts): sound byte under Built-in video, and alert audio that reaches the device

Creator request: a sound-byte browse + label under Built-in video, with an option to loop the
sound while the clip plays. The video itself is always one pass to completion.

- Source gains AlertAudioPath and AlertAudioLoop, persisted (column + migration + INSERT +
  SELECT + reader, the whole chain — that chain is where the blur crashed once).
- The sound byte plays through MediaPlayer on the default output device; MediaEnded re-seeks to
  zero while looping, and StopClip kills it so a tail cannot bleed into the next alert.
- The clip's OWN soundtrack moved off the mixer and onto the device as well
  (Services/Audio/AlertAudioOutput: one WaveOutEvent + buffered float provider).

Why the move: the creator asked that alert audio — the clip's track or the added sound byte —
appear in the desktop sound bar. Only audio that reaches the default device is picked up by the
WASAPI loopback that feeds both the meter and the stream. The old EnqueueAlertAudio route put the
sound in the stream but left the meter flat and the room silent.

Tests: the sound byte and its loop flag round-trip through SQLite, and a fresh alert layer starts
with no sound byte and no loop. Release 0 warnings.

Models/Source.cs

@@ -273,6 +273,28 @@ public class Source : SceneElement
         set => Set(ref _alertVideoVolume, Math.Clamp(value, 0.0, 1.0));
     }
 
+    // Sound byte that plays with the alert clip (creator ruling 2026-11-01). Referenced from disk
+    // like the clip; the video always plays once to completion, the sound may loop under it.
+    private string? _alertAudioPath;
+    public string? AlertAudioPath
+    {
+        get => _alertAudioPath;
+        set => Set(ref _alertAudioPath, value);
+    }
+
+    private bool _alertAudioLoop;
+    public bool AlertAudioLoop
+    {
+        get => _alertAudioLoop;
+        set => Set(ref _alertAudioLoop, value);
+    }
+
+    public string AlertAudioFileDisplay =>
+        string.IsNullOrWhiteSpace(_alertAudioPath) ? "No sound byte" : System.IO.Path.GetFileName(_alertAudioPath);
+
+    /// <summary>True once a file is chosen — gates the loop switch and the clear button.</summary>
+    public bool HasAlertAudio => !string.IsNullOrWhiteSpace(_alertAudioPath);
+
     private AlertDisplayMethod _alertDisplayMethod = AlertDisplayMethod.TickerScroll;
     public AlertDisplayMethod AlertDisplayMethod
     {