fix(scoreboard): the strip and the social bar now really share the frame

Reported: the ticker was painting OVER the socials. The coupling rule existed and was tested
(ScoreboardLayout.ResolveEdges) but nothing ever CALLED it — the strip's Y still came from its own
preference, so both bars landed on the south edge.

Now the resolution runs at publish time: the social bar keeps its side, the scoreboard takes the
other one, and its Y follows from that edge. The pair re-sorts whenever the social bar moves (the
preview click-toggle) or shows/hides, because the rule is "resolve from what is visible", not a
stored pairing.

The layer also gains an Edge preference (Top/Bottom) in its properties, labelled "socials take the
other" — a preference, never a claim on an occupied edge.

Also stabilised the opacity assertion: it samples a plate-only pixel 5px from the left edge instead
of the first semi-transparent pixel it finds (which could be anti-aliased text).

Controls/LeftPanel.xaml

@@ -473,6 +473,12 @@
                                 <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="Edge (socials take the other)" Foreground="{StaticResource Brush.OnSurfaceVariant}"
+                                           FontSize="11" Margin="10,0,0,2"/>
+                                <ComboBox Style="{StaticResource YtComboBox}" Height="24" FontSize="11" Margin="10,0,0,6"
+                                          ItemsSource="{Binding ScoreboardEdgeOptions}"
+                                          SelectedItem="{Binding ScoreboardEdge, Mode=TwoWay}"/>
+
                                 <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"