feat(scoreboard): a master on/off switch that also stops the polling

Creator: "even if it's not visible, we want to avoid polling."

ScoreboardEnabled is the layer's master switch in the properties panel. Off means two things:
- the strip frame is null in every scene (per-scene eye toggles stay independent), and
- the cadence policy returns None at every phase — a scoreboard nobody can see must not spend quota.

The switch is persisted (Scoreboard.Enabled), because one that resets would quietly resume polling.

The polling gate is ScoreboardPollsEnabled: master switch AND at least one scene with a visible
scoreboard — so hiding it in all five scenes also stops the polls.

Guards: ScoreboardCadenceTests adds the switch off across live/test/recording; ScoreboardProperties
pins the checkbox and the VM surface.

ViewModels/MainViewModel.Scoreboard.cs

@@ -28,6 +28,28 @@ public partial class MainViewModel
 
     internal ScoreboardRecorder ScoreboardRecorder => _scoreboard;
 
+    private bool _scoreboardEnabled = true;
+
+    /// <summary>The scoreboard's master switch. Off hides the strip in EVERY scene (the per-scene
+    /// eye stays independent) and — the point of the switch — stops all polling: a scoreboard nobody
+    /// can see must not spend quota.</summary>
+    public bool ScoreboardEnabled
+    {
+        get => _scoreboardEnabled;
+        set
+        {
+            if (!SetProperty(ref _scoreboardEnabled, value)) return;
+            _layoutStore.SaveScoreboardEnabled(value);
+            RefreshScoreboardPreviews();
+            OnPropertyChanged(nameof(ScoreboardPollsEnabled));
+        }
+    }
+
+    /// <summary>What the cadence policy asks: the master switch AND at least one scene showing it.</summary>
+    internal bool ScoreboardPollsEnabled =>
+        _scoreboardEnabled && Scenes.Any(scene =>
+            scene.Elements.OfType<Source>().Any(s => s.IsScoreboard && s.IsVisible && s.IsEnabled));
+
     internal ScoreboardTiles ScoreboardTilesState => _scoreboardTiles;
 
     /// <summary>Composited strip frame for the output path (cached by the presenter).</summary>
@@ -146,7 +168,7 @@ public partial class MainViewModel
     /// must never touch a WriteableBitmap, so this runs on the UI thread only.</summary>
     private void RefreshScoreboardPreviews()
     {
-        var frame = _scoreboardPresenter?.Frame;
+        var frame = _scoreboardEnabled ? _scoreboardPresenter?.Frame : null;
         foreach (var scene in Scenes)
         foreach (var source in scene.Elements.OfType<Source>().Where(s => s.IsScoreboard))
         {