feat(scoreboard): scroll speed is a property (20-400 px/s, persisted)

The marquee now has the dial it needs. `ScoreboardPresenter.PixelsPerSecond` is the single owner of
the range (20-400, default 120), the layer's properties expose it with a live px/s readout, and the
value is persisted as Scoreboard.ScrollSpeed because it is a READABILITY choice — re-dialling it
every launch would be a tax on the creator who needs the strip to be readable.

Advance() still takes an optional override so tests can drive exact distances.

Guard: the presenter test proves speed is real (same elapsed time, twice the speed, different
window) — not a value that is stored and ignored — and the panel test pins the control, its range
and the persistence key.

ViewModels/MainViewModel.Scoreboard.cs

@@ -43,6 +43,20 @@ public partial class MainViewModel
     }
 
     public IReadOnlyList<BarEdge> ScoreboardEdgeOptions { get; } = new[] { BarEdge.Top, BarEdge.Bottom };
+
+    /// <summary>How fast the ticker travels, in pixels per second. Persisted like the rest of the
+    /// strip's look: it is a readability choice, and re-dialling it every launch would be a tax.</summary>
+    public double ScoreboardScrollSpeed
+    {
+        get => _scoreboardPresenter?.PixelsPerSecond ?? ScoreboardPresenter.DefaultPixelsPerSecond;
+        set
+        {
+            var clamped = Math.Clamp(value, ScoreboardPresenter.MinPixelsPerSecond, ScoreboardPresenter.MaxPixelsPerSecond);
+            if (_scoreboardPresenter != null) _scoreboardPresenter.PixelsPerSecond = clamped;
+            _layoutStore.SaveScoreboardScrollSpeed((int)Math.Round(clamped));
+            OnPropertyChanged(nameof(ScoreboardScrollSpeed));
+        }
+    }
     private Source? _scoreboardPreviewSource;
 
     internal ScoreboardRecorder ScoreboardRecorder => _scoreboard;