feat(scoreboard): a DEBUG-only "Simulate" switch, so the strip is provable in the preview

Creator (2026-10-30): pre-1.0, use DEBUG — add a toggle that simulates the ticker so the strip can
be SEEN and its compositing verified before a real stream.

The sample travels the REAL ScoreboardRecorder a stream uses (one new member, a gift of three, a
milestone, a $12.34 Super Chat), so what the preview shows is exactly what live events produce — a
fake renderer would prove nothing. Toggling off clears to an empty session, so the empty state is
visible too.

Debug-gated at COMPILE time: `DevelopmentBuild` is a const true in DEBUG / false in Release, and the
checkbox binds its Visibility to it. A shipped Release build therefore has no dev control at all,
rather than one hidden by a runtime flag.

Guard: ScoreboardSimulationTests — the sample lands all four tallies through the recorder, off clears,
and the panel + const pair exist in the Dual (DEBUG/Release) form.

Controls/LeftPanel.xaml

@@ -460,6 +460,12 @@
                                 <TextBlock Text="Sign in to YouTube to use the scoreboard."
                                            Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="0,0,0,4"
                                            Visibility="{Binding IsConnected, Converter={StaticResource InverseBoolToVis}}"/>
+                                <!-- 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}"
+                                          ToolTip="Push sample events so the strip appears in the preview."
+                                          Visibility="{Binding IsDevelopmentBuild, Converter={StaticResource BoolToVis}}"/>
                                 <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."/>