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).

ytLive.Tests/ScoreboardSimulationTests.cs

@@ -43,7 +43,6 @@ public class ScoreboardSimulationTests
         Assert.NotNull(type.GetProperty("ScoreboardSimulate"));
         Assert.NotNull(type.GetProperty("IsDevelopmentBuild"));
         // DEBUG-gated at compile time, so a Release build has no dev control at all.
-        var vmSource = SourceAssert.Text("ViewModels", "MainViewModel.Scoreboard.cs");
         Assert.Contains("#if DEBUG", vmSource);
         Assert.Contains("private const bool DevelopmentBuild = true;", vmSource);
         Assert.Contains("private const bool DevelopmentBuild = false;", vmSource);