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.

ytLive.Tests/ScoreboardLayerTests.cs

@@ -25,6 +25,12 @@ public class ScoreboardLayerTests
         var resolver = SourceAssert.Text("ViewModels", "MainViewModel.cs");
         Assert.Contains("Source { Type: SourceType.Scoreboard } => _scoreboardPresenter?.Frame", resolver);
 
+        // The PREVIEW must show the strip: its frame is a runtime frame-source and its Image needs
+        // the visibility trigger. Missing either made every property change look inert (2026-10-30).
+        var source = SourceAssert.Text("Models", "Source.cs");
+        Assert.Contains("or SourceType.AlertBox or SourceType.Scoreboard", source);
+        Assert.Contains("IsScoreboard", SourceAssert.Text("Controls", "PreviewPane.xaml"));
+
         // Session lifecycle: every start clears the tallies.
         var ops = SourceAssert.Text("ViewModels", "MainViewModel.Streaming.Operations.cs");
         Assert.Contains("StartScoreboardSession();", ops);