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.

Models/ScoreboardCadence.cs

@@ -29,8 +29,13 @@ public static class ScoreboardCadence
     public static readonly TimeSpan AnalyticsInterval = TimeSpan.FromMinutes(30);
     public static readonly TimeSpan SubscriberInterval = TimeSpan.FromSeconds(60);
 
-    public static ScoreboardPoll Due(SessionPhase phase, DateTime lastAnalyticsUtc, DateTime lastSubscriberUtc, DateTime nowUtc)
+    /// <summary><paramref name="enabled"/> is the layer's master switch. Off means NOTHING is
+    /// polled, whatever the session is doing — a hidden scoreboard has no reason to spend quota
+    /// (creator ruling 2026-10-30).</summary>
+    public static ScoreboardPoll Due(SessionPhase phase, DateTime lastAnalyticsUtc, DateTime lastSubscriberUtc,
+        DateTime nowUtc, bool enabled = true)
     {
+        if (!enabled) return ScoreboardPoll.None;
         if (phase == SessionPhase.Idle) return ScoreboardPoll.None;
 
         var analyticsDue = phase == SessionPhase.Live && nowUtc - lastAnalyticsUtc >= AnalyticsInterval;