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.

Controls/LeftPanel.xaml

@@ -463,9 +463,13 @@
                                 <!-- DEBUG builds only: prove the strip in the preview before a real
                                      stream. Absent from Release, not merely hidden by a flag. -->
                                 <CheckBox Style="{StaticResource YtCheckBox}" Margin="0,0,0,4"
-                                          Content="Simulate (dev)" IsChecked="{Binding ScoreboardSimulate, Mode=TwoWay}"
+                                          IsChecked="{Binding ScoreboardSimulate, Mode=TwoWay}"
                                           ToolTip="Push sample events so the strip appears in the preview."
-                                          Visibility="{Binding IsDevelopmentBuild, Converter={StaticResource BoolToVis}}"/>
+                                          Visibility="{Binding IsDevelopmentBuild, Converter={StaticResource BoolToVis}}">
+                                    <!-- Dev-only affordances wear orange so a development control can never
+                                         be mistaken for product. -->
+                                    <TextBlock Text="Simulate (dev)" Foreground="{StaticResource Brush.Attention}" FontSize="12"/>
+                                </CheckBox>
                                 <CheckBox Style="{StaticResource YtCheckBox}" Margin="0,0,0,4"
                                           Content="Show the scoreboard" IsChecked="{Binding ScoreboardEnabled, Mode=TwoWay}"
                                           ToolTip="Off hides the strip in every scene AND stops the channel polling."/>

Controls/PreviewPane.xaml

@@ -138,6 +138,11 @@
                                                                      <DataTrigger Binding="{Binding IsAlertBox}" Value="True">
                                                                          <Setter Property="Visibility" Value="Visible"/>
                                                                      </DataTrigger>
+                                                                     <!-- The scoreboard strip is a frame-source too: without this trigger its
+                                                                          Image stays collapsed and every property change looks inert. -->
+                                                                     <DataTrigger Binding="{Binding IsScoreboard}" Value="True">
+                                                                         <Setter Property="Visibility" Value="Visible"/>
+                                                                     </DataTrigger>
                                                                      <DataTrigger Binding="{Binding IsWebSource}" Value="True">
                                                                          <Setter Property="Visibility" Value="Visible"/>
                                                                          <Setter Property="Stretch" Value="Fill"/>

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;
 

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);

ytLive.Tests/ScoreboardPropertiesTests.cs

@@ -25,6 +25,10 @@ public class ScoreboardPropertiesTests
         Assert.Contains("Sign in to YouTube to use the scoreboard.", block);
         Assert.Contains("CanUseScoreboard, Converter={StaticResource InverseBoolToVis}", block);
 
+        // Dev-only labels are ORANGE, so a development affordance cannot pass for product.
+        Assert.Contains("Simulate (dev)", block);
+        Assert.Contains("Brush.Attention", block);
+
         // The master switch sits in the same panel: off hides the strip and stops the polling.
         Assert.Contains("ScoreboardEnabled", panel);