fix(theater): auto-exit Theater Mode when both visualizer subsystems are disabled

Adds CoerceTheaterMode() to WaveformVisualizerControlState; ToggleLava/ToggleWaveform
call it before NotifyChanged so all observers see consistent state in one Changed cycle.
Covers the dead-end escape route bug (Phase 20 review finding).

DeepDrftPublic.Client/Controls/WaveformVisualizerControls.razor

@@ -226,12 +226,14 @@
     private void ToggleLava()
     {
         ControlState.LavaEnabled = !ControlState.LavaEnabled;
+        ControlState.CoerceTheaterMode();
         ControlState.NotifyChanged();
     }
 
     private void ToggleWaveform()
     {
         ControlState.WaveformEnabled = !ControlState.WaveformEnabled;
+        ControlState.CoerceTheaterMode();
         ControlState.NotifyChanged();
     }
 

DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs

@@ -160,6 +160,19 @@ public sealed class WaveformVisualizerControlState
     /// </summary>
     public event Action? Changed;
 
+    /// <summary>
+    /// Enforces the Theater-Mode invariant: Theater Mode cannot remain on when both visualizer
+    /// subsystems are off (there is nothing to go to theater FOR). Call this after mutating
+    /// <see cref="LavaEnabled"/> or <see cref="WaveformEnabled"/> and before
+    /// <see cref="NotifyChanged"/> so all observers see a consistent, coerced state in the same
+    /// <see cref="Changed"/> cycle.
+    /// </summary>
+    public void CoerceTheaterMode()
+    {
+        if (TheaterMode && !LavaEnabled && !WaveformEnabled)
+            TheaterMode = false;
+    }
+
     /// <summary>Raise <see cref="Changed"/>. Called by the controls component after mutating a value.</summary>
     public void NotifyChanged() => Changed?.Invoke();
 }