fix(scoreboard): the strip finally reaches the preview, and dev labels are orange

"Clicking on everything produces no visible effect" — the cause was upstream of the controls, and it
is the same defect as the invisible alert box (TASK 47):

- `Source.DisplaySource` returned `VideoImageSource` only for ChatBox / WebSource / MediaSource /
  AlertBox. The scoreboard's frame was rendered and published, then never displayed.
- the preview's Image has a per-type Visibility trigger for each of those and none for IsScoreboard,
  so even a correct DisplaySource would have stayed collapsed.

Both are fixed, and the layer test now pins the frame-source list AND the trigger so the pair cannot
drift apart again.

Dev-only affordances are ORANGE from now on (creator ruling): "Simulate (dev)" is a TextBlock in
Brush.Attention, so a development control can never be mistaken for product. The panel test pins the
colour.

Models/Source.cs

@@ -143,9 +143,16 @@ public class Source : SceneElement
 
     public ImageSource? ImageSource => _imageSource;
 
+    /// <summary>Types whose frame is produced at runtime rather than stored as an asset. The
+    /// scoreboard joined this list 2026-10-30 — without it the strip was rendered, published to
+    /// VideoImageSource, and never displayed (the same shape as the invisible-alert defect).</summary>
+    private bool IsFrameSource =>
+        Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource
+             or SourceType.AlertBox or SourceType.Scoreboard;
+
     public override ImageSource? DisplaySource => IsLiveCapture
         ? (VideoImageSource ?? _imageSource)
-        : (Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource or SourceType.AlertBox ? VideoImageSource : _imageSource);
+        : (IsFrameSource ? VideoImageSource : _imageSource);
 
     public override bool IsImageSource => Type == SourceType.Image;