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.

Models/ScoreboardLayout.cs

@@ -53,11 +53,12 @@ public static class ScoreboardLayout
     public static Fit FitLine(IReadOnlyList<double> tileWidths, double separatorWidth,
         double availableWidth, double requestedSize = MaxFontSize, double floorSize = FloorFontSize)
     {
-        var count = tileWidths?.Count ?? 0;
+        var widths = tileWidths ?? System.Array.Empty<double>();
+        var count = widths.Count;
         if (count == 0 || availableWidth <= 0)
             return new Fit(requestedSize, 1, false);
 
-        var total = tileWidths!.Sum() + separatorWidth * (count - 1);
+        var total = widths.Sum() + separatorWidth * (count - 1);
         if (total <= availableWidth)
             return new Fit(requestedSize, 1, false);
 
@@ -70,7 +71,7 @@ public static class ScoreboardLayout
         var factor = floorSize / requestedSize;
         var lines = 1;
         var lineWidth = 0.0;
-        foreach (var width in tileWidths)
+        foreach (var width in widths)
         {
             var w = width * factor;
             var needed = lineWidth == 0 ? w : lineWidth + separatorWidth + w;

ViewModels/MainViewModel.cs

@@ -596,7 +596,7 @@ public partial class MainViewModel : ViewModelBase
             WebcamSceneConfig webcam => _cameraManager.GetLatestFrame(DeviceKeyForWebcam(webcam.WebcamId), EffectKeyFor(webcam)),
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
-            Source { Type: SourceType.Scoreboard } => _scoreboardPresenter.Frame,
+            Source { Type: SourceType.Scoreboard } => _scoreboardPresenter?.Frame,
             Source { Type: SourceType.AlertBox } alertBox => RenderAlertBox(alertBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),
             Source { IsMediaSource: true, MediaPath: not null } media => _mediaManager.GetLatestFrame(media.MediaPath),

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");