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.

ytLive.Tests/ScoreboardCadenceTests.cs

@@ -32,6 +32,11 @@ public class ScoreboardCadenceTests
         Assert.Equal(ScoreboardPoll.None,
             ScoreboardCadence.Due(SessionPhase.Live, old, now.AddSeconds(-10), now));
 
+        // The master switch beats every other rule: off means silent, whatever the session is.
+        Assert.Equal(ScoreboardPoll.None, ScoreboardCadence.Due(SessionPhase.Live, old, old, now, enabled: false));
+        Assert.Equal(ScoreboardPoll.None, ScoreboardCadence.Due(SessionPhase.Test, old, old, now, enabled: false));
+        Assert.Equal(ScoreboardPoll.None, ScoreboardCadence.Due(SessionPhase.Recording, old, old, now, enabled: false));
+
         // Subscribers alone mid-window.
         Assert.Equal(ScoreboardPoll.SubscribersOnly,
             ScoreboardCadence.Due(SessionPhase.Live, now, old, now));