fix(scoreboard): box follows the rendered strip, opacity proven, plate always settable, dev override

Four reports, three causes:

1. "Background plate has no code behind it" — the plate COLOUR and OPACITY controls were disabled
   until the plate switch was on, which reads as dead controls. They are always settable now; the
   switch is what decides whether a plate is drawn.
2. "Opacity doesn't work" — proven in the renderer test: 20% and 80% produce alphas of ~51 and ~204,
   so the slider reaches the pixels. What was actually wrong was the box.
3. The strip's BOX was fixed at 56px while a wrapped strip renders taller — the frame was scaled
   into the box and looked squashed/clipped. The source's width/height now follow the rendered
   frame, and its Y follows from the edge.
4. "Simulate (dev) should override Show the scoreboard" — it does: the preview gate is
   (_scoreboardEnabled || _scoreboardSimulate), so the dev toggle works even with the product
   setting off.

Not changed: the strip is STATIC by design (all enabled facts always visible, wrap rather than
scroll). If scrolling is wanted after all, that is a presentation mode to add, not a fix.

ytLive.Tests/ScoreboardSimulationTests.cs

@@ -29,6 +29,11 @@ public class ScoreboardSimulationTests
         recorder.StartSession();
         Assert.Equal(0, recorder.State.NewMembers);
 
+        // Simulate OVERRIDES the master "show" switch: the dev toggle must work even when the
+        // product setting is off.
+        var vmSource = SourceAssert.Text("ViewModels", "MainViewModel.Scoreboard.cs");
+        Assert.Contains("_scoreboardEnabled || _scoreboardSimulate", vmSource);
+
         // The control exists and is gated on the build, not on a runtime flag.
         var panel = SourceAssert.Text("Controls", "LeftPanel.xaml");
         Assert.Contains("ScoreboardSimulate", panel);