TASK 47: draw the alert clip — the preview Image was Collapsed for AlertBox

Four builds (89/90 + two) burned proving the alert video was PERFECT: real h264
1280x720, 240 frames decoded, alert audio in the live mix, and the shipped asset
byte-identical (md5 0ee1f496…) to the creator's llamacasty-dancingLlama-thankyou.mp4
with every sampled frame full bright content. Build 90's new diagnostics then showed
the frame reaching BOTH consumers every second for the whole clip —
`Alert preview: frame=680x200 a255` and `Output resolver: frame 680x200
playing=True` — an opaque, correctly-sized frame, handed over and never seen.

Root cause: Controls/PreviewPane.xaml keeps the per-element Image Collapsed unless a
DataTrigger fires, and there was no IsAlertBox trigger (only IsImageSource / IsChatBox /
IsWebSource / IsWebcam). The alert box's Image was therefore Collapsed forever. Chat
boxes rendered because they HAVE a trigger — that asymmetry is the whole clue, and it
is why removing/re-adding the layer could never fix it.

Fix: an IsAlertBox DataTrigger beside the IsChatBox one.

Two Good Dog tests, because the existing fakes had been hiding this:
- AlertBoxPreviewVisibilityTests (red `Expected: Visible / Actual: Collapsed`, now
  green) drives the real MainWindow + PreviewPane and asserts the bound Image is
  Visible — the first alert test that crosses the XAML at all.
- AlertClipOutputTests is the first test in the repo to run a REAL codec: pinned ffmpeg
  generates a clip, the real AlertClipDecoder + AlertOverlayLayer + SceneCompositor
  composite it, and the box rect must be a colourful picture that DIFFERS from idle. It
  passed while the feature was broken in the app — which is exactly why it was needed:
  it exonerated decode+layer+compositor and pointed the hunt at the last hop.

Also keeps the two once-per-second alert diagnostics (resolver + preview) that settled
it, pending the creator's call on whether to keep them.

Notes: the pinned BtbN ffmpeg has no libx264, so the test clip is -c:v mpeg4.
SceneCompositor.Render fills its base with opaque black, so the test compares against an
idle render rather than counting non-zero bytes. MediaSource has the same missing trigger
in PreviewPane.xaml — left unfixed as out of scope, noted in HANDOFF.

Full suite 324/324. Lesson recorded in MyMistakes.md rule (5): a producer that hands over
a correct frame has still delivered nothing — when output arrives and the user still sees
nothing, stop auditing the producer and audit the consumer's VISIBILITY.

ViewModels/MainViewModel.cs

@@ -508,9 +508,25 @@ public partial class MainViewModel : ViewModelBase
                 AppLog.Write($"Output resolver: webcam '{webcam.WebcamId}' frame is null — layer skipped in composite");
             }
         }
+        // Alerts (TASK 47, 2026-09-26): a healthy decode logged frames=240 while the
+        // box stayed blank, so name the two possibilities per tick — did the resolver
+        // ever get asked for the alert box, and did it have a frame to hand back?
+        if (element is Source { Type: SourceType.AlertBox } alert)
+        {
+            var now = DateTime.UtcNow;
+            if (now - _lastAlertResolveLog >= TimeSpan.FromSeconds(1))
+            {
+                _lastAlertResolveLog = now;
+                AppLog.Write(frame == null
+                    ? $"Output resolver: alert box '{alert.Id}' frame is null (visible={alert.IsVisible} playing={_alertLayer.IsPlaying}) — skipped in composite"
+                    : $"Output resolver: alert box '{alert.Id}' frame {frame!.Width}x{frame!.Height} (visible={alert.IsVisible} playing={_alertLayer.IsPlaying})");
+            }
+        }
         return frame;
     }
 
+    private DateTime _lastAlertResolveLog = DateTime.MinValue;
+
     private DateTime _lastWebcamNullLog = DateTime.MinValue;
 
     /// <summary>Maps a config's identity GUID to the CameraManager session key (the