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.cs

@@ -280,6 +280,7 @@ public partial class MainViewModel : ViewModelBase
         _musicPlayer.UserGain = (float)Helpers.GainDb.FromDb(TraxDefaultDb);   // background by default
         InitScoreboard();
         ScoreboardEnabled = _layoutStore.LoadScoreboardEnabled();
+        ScoreboardScrollSpeed = _layoutStore.LoadScoreboardScrollSpeed();
         try
         {
             var s = _layoutStore.GetSetting("ZoomShape");