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.

Services/Compositor/ScoreboardPresenter.cs

@@ -31,13 +31,21 @@ public sealed class ScoreboardPresenter
     /// creator previews is what ships.</summary>
     public bool Scroll { get; set; } = true;
 
+    /// <summary>Marquee speed in pixels per second — the creator's dial (a read-speed choice, not a
+    /// styling one).</summary>
+    public double PixelsPerSecond { get; set; } = DefaultPixelsPerSecond;
+
+    public const double DefaultPixelsPerSecond = 120;
+    public const double MinPixelsPerSecond = 20;
+    public const double MaxPixelsPerSecond = 400;
+
     /// <summary>Advances the scroll by <paramref name="seconds"/>. The strip is a marquee: one
     /// measured run, cropped to a moving window, so a tick is a row-wise byte copy rather than a
     /// re-rasterisation.</summary>
-    public void Advance(double seconds, double pixelsPerSecond = 120)
+    public void Advance(double seconds, double? pixelsPerSecond = null)
     {
         if (!Scroll) return;
-        _offsetPx += seconds * pixelsPerSecond;
+        _offsetPx += seconds * (pixelsPerSecond ?? PixelsPerSecond);
     }
 
     /// <summary>The strip frame for the current tallies, or null when nothing is enabled/shown.</summary>