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).

ViewModels/MainViewModel.Socials.cs

@@ -224,6 +224,7 @@ public partial class MainViewModel
     private void NotifySocialsChanged()
     {
         RenderSocialBarFrame();
+        RefreshScoreboardPreviews();   // the pair re-sorts when the social bar shows/hides
         OnPropertyChanged(nameof(SocialBarTextBrush));
         OnPropertyChanged(nameof(SocialBarBackgroundBrush));
         OnPropertyChanged(nameof(HasSocials));
@@ -238,6 +239,8 @@ public partial class MainViewModel
         if (_socials == null || _socials.BarPosition == position) return;
         _socials.BarPosition = position;
         OnPropertyChanged(nameof(SocialBarTop));
+        // Coupled pair: moving the social bar moves the scoreboard to the other edge.
+        RefreshScoreboardPreviews();
         ScheduleSave();
     }