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/LayoutStore.Settings.cs

@@ -1,4 +1,5 @@
 using Microsoft.Data.Sqlite;
+using ytLive.Services.Compositor;
 using ytLive.Models;
 
 namespace ytLive.Services;
@@ -57,6 +58,15 @@ public partial class LayoutStore : IDisposable
 
     public void SaveScoreboardEnabled(bool enabled) => UpsertSetting("Scoreboard.Enabled", enabled ? "1" : "0");
 
+    /// <summary>Scoreboard marquee speed (px/s), restored at startup.</summary>
+    public int LoadScoreboardScrollSpeed()
+        => int.TryParse(GetSetting("Scoreboard.ScrollSpeed"), out var px)
+            ? Math.Clamp(px, (int)ScoreboardPresenter.MinPixelsPerSecond, (int)ScoreboardPresenter.MaxPixelsPerSecond)
+            : (int)ScoreboardPresenter.DefaultPixelsPerSecond;
+
+    public void SaveScoreboardScrollSpeed(int pixelsPerSecond)
+        => UpsertSetting("Scoreboard.ScrollSpeed", pixelsPerSecond.ToString());
+
     /// <summary>The user's chosen record output folder (TASK 18), or null to use the
     /// default <c>%APPDATA%\ytLlive\recordings</c>. Restored at startup.</summary>
     public string? LoadRecordFolder()