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"/>