feat(35): scenes remember the room — BRB mutes the mic on entry

591 green, 0 warnings.

- `Scene.SceneMutesMic` / `SceneMutesDesktop` are **nullable booleans**: null = leave the mix alone, so
  a scene is passive until the creator says otherwise. Schema columns added, NULL for every existing
  layout except one deliberate default.
- **Applied once when the scene takes the PROGRAM output** (`TransitionToScene`, only when live or
  recording). Staging a scene offline is auditioning, not a stream edit — the test asserts that.
- **The creator is boss:** a manual mute survives any transition into a scene with no opinion; a scene
  that does have one wins on its next entry. `SetMicMuted`/`SetGameMuted` are idempotent so a repeated
  "mute" cannot clobber the level that unmute would restore.
- **BRB ships muted** (creator spec) for new layouts *and* via a one-time migration for existing rows
  where the creator has never expressed an opinion.
- UI: two three-state checkboxes under the layers list ("When this scene goes live"), styled with
  `PropertyPillCheck` so the theme test holds.

`SceneAudioTests` covers the whole rule in one integration test: default state, offline staging does
nothing, live entry mutes, manual unmute survives a passive scene, an opinionated scene wins, desktop
untouched, and all three states roundtrip through save/reload.

Needs the creator's eye: the checkboxes are new chrome in the left panel (indeterminate = passive).

ViewModels/MainViewModel.Audio.cs

@@ -294,6 +294,38 @@ public partial class MainViewModel
     /// lip-sync is right", so it deserves one click, not a drag.</summary>
     internal void SetSyncOffset(int milliseconds) => AudioSyncOffsetMs = milliseconds;
 
+    /// <summary>TASK 35: mute/unmute from a scene entry. Idempotent — a scene that says "mute" while
+    /// the mic is already muted must not overwrite the level it would restore.</summary>
+    internal void SetMicMuted(bool muted)
+    {
+        if (muted == MicMuted) return;
+        if (muted)
+        {
+            _volumeBeforeMute = MicVolume;
+            MicVolume = 0;
+        }
+        else
+        {
+            MicVolume = _volumeBeforeMute ?? 0.8;
+            BeginVolumeFlash();
+        }
+    }
+
+    /// <summary>TASK 35: as <see cref="SetMicMuted"/>, for desktop/game audio.</summary>
+    internal void SetGameMuted(bool muted)
+    {
+        if (muted == GameMuted) return;
+        if (muted)
+        {
+            _gameVolumeBeforeMute = GameAudioVolume;
+            GameAudioVolume = 0;
+        }
+        else
+        {
+            GameAudioVolume = _gameVolumeBeforeMute ?? 1.0;
+        }
+    }
+
     private void ToggleMicMute()
     {
         if (MicMuted)

ViewModels/MainViewModel.Scenes.cs

@@ -21,6 +21,32 @@ public partial class MainViewModel
     // XAML bindings and MainViewModel call sites keep working unchanged.
     public ObservableCollection<Scene> Scenes => _sceneGraph.Scenes;
 
+    /// <summary>TASK 35: the staged scene's audio state, bound by the left panel's three-state
+    /// checkboxes (indeterminate = leave the mix alone).</summary>
+    public bool? SceneMutesMic
+    {
+        get => StagedScene?.SceneMutesMic;
+        set
+        {
+            if (StagedScene is not { } scene || scene.SceneMutesMic == value) return;
+            scene.SceneMutesMic = value;
+            ScheduleSave();
+            OnPropertyChanged(nameof(SceneMutesMic));
+        }
+    }
+
+    public bool? SceneMutesDesktop
+    {
+        get => StagedScene?.SceneMutesDesktop;
+        set
+        {
+            if (StagedScene is not { } scene || scene.SceneMutesDesktop == value) return;
+            scene.SceneMutesDesktop = value;
+            ScheduleSave();
+            OnPropertyChanged(nameof(SceneMutesDesktop));
+        }
+    }
+
     public Scene? StagedScene
     {
         get => _stagedScene;
@@ -38,6 +64,8 @@ public partial class MainViewModel
                 // (build 1082: the option vanished after the first layout load).
                 OnPropertyChanged(nameof(CanAddBlur));
                 OnPropertyChanged(nameof(StagedBrandLogo));
+                OnPropertyChanged(nameof(SceneMutesMic));
+                OnPropertyChanged(nameof(SceneMutesDesktop));
 
                 SelectedElement = null;
                 if (value != null)
@@ -137,9 +165,24 @@ public partial class MainViewModel
         {
             LiveScene = scene;
             _transition.Start(SelectedTransitionType, TransitionDurationMs, null);
+            // TASK 35: the scene carries the room. Applied ONCE at entry, and only when the program
+            // output actually changes — staging a scene offline must never edit the stream's state.
+            ApplySceneAudio(scene);
         }
     }
 
+    /// <summary>TASK 35: a scene's audio state, applied once when it takes the program output.
+    /// <c>null</c> means "leave the mix alone" — the creator's manual mute survives until a scene that
+    /// actually has an opinion takes over.</summary>
+    private void ApplySceneAudio(Scene scene)
+    {
+        if (scene.SceneMutesMic is { } muteMic)
+            SetMicMuted(muteMic);
+
+        if (scene.SceneMutesDesktop is { } muteDesktop)
+            SetGameMuted(muteDesktop);
+    }
+
     /// <summary>Raised before a scene switch so the preview can animate the old picture away.</summary>
     public event Action<TransitionType, double>? PreviewTransition;
 
@@ -377,6 +420,8 @@ public partial class MainViewModel
             Name = name.Trim(),
             IsChatScene = SceneCatalog.IsChat(name),
             HasBackground = SceneCatalog.HasBackground(name),
+            // TASK 35: BRB ships with the mic muted; every other scene is passive until told.
+            SceneMutesMic = SceneCatalog.Is(name, SceneCatalog.Brb) ? true : null,
         };
         EnsureBackground(scene);
         EnsureZoomLayer(scene);