fix(scoreboard): clear the two nullable warnings

Both were mine, from the scoreboard units:
- ScoreboardLayout.FitLine null-forgave tileWidths on one expression and then iterated the field
  anyway; it now binds a non-null local once and uses that throughout.
- the output resolver dereferenced _scoreboardPresenter, which is assigned in InitScoreboard — it
  is nullable by construction, so the resolver reads '?.' exactly like the other optional services.

Clean build, 0 warnings, scoreboard tests green.

ytLive.Tests/ScoreboardLayerTests.cs

@@ -23,7 +23,7 @@ public class ScoreboardLayerTests
 
         // The output resolver knows how to produce its frame (same shape as the chat box/alert box).
         var resolver = SourceAssert.Text("ViewModels", "MainViewModel.cs");
-        Assert.Contains("Source { Type: SourceType.Scoreboard } => _scoreboardPresenter.Frame", resolver);
+        Assert.Contains("Source { Type: SourceType.Scoreboard } => _scoreboardPresenter?.Frame", resolver);
 
         // Session lifecycle: every start clears the tallies.
         var ops = SourceAssert.Text("ViewModels", "MainViewModel.Streaming.Operations.cs");