feat(scoreboard): a master on/off switch that also stops the polling

Creator: "even if it's not visible, we want to avoid polling."

ScoreboardEnabled is the layer's master switch in the properties panel. Off means two things:
- the strip frame is null in every scene (per-scene eye toggles stay independent), and
- the cadence policy returns None at every phase — a scoreboard nobody can see must not spend quota.

The switch is persisted (Scoreboard.Enabled), because one that resets would quietly resume polling.

The polling gate is ScoreboardPollsEnabled: master switch AND at least one scene with a visible
scoreboard — so hiding it in all five scenes also stops the polls.

Guards: ScoreboardCadenceTests adds the switch off across live/test/recording; ScoreboardProperties
pins the checkbox and the VM surface.

Controls/LeftPanel.xaml

@@ -453,6 +453,9 @@
                             <!-- Scoreboard config (creator 2026-10-30): the ego wall's billboard —
                                  which eight facts show, and how the strip is painted. -->
                             <StackPanel Visibility="{Binding SelectedElement.IsScoreboard, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
+                                <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."/>
                                 <TextBlock Text="Facts" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" FontWeight="SemiBold" Margin="0,0,0,4"/>
                                 <WrapPanel Margin="10,0,0,0">
                                     <CheckBox Style="{StaticResource YtCheckBox}" Content="Subscribers" Margin="0,0,10,4"