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).

Models/Scene.cs

@@ -72,6 +72,28 @@ public class Scene : INotifyPropertyChanged
 
     public bool IsChatScene { get; init; }
 
+    private bool? _sceneMutesMic;
+
+    /// <summary>TASK 35: what this scene does to the mix when it takes the program output.
+    /// <c>null</c> = leave it alone (the default; scenes are passive until told); true mutes the mic on
+    /// entry. Applied once at entry, never while staging offline — auditioning a scene must not edit
+    /// the stream's state.</summary>
+    public bool? SceneMutesMic
+    {
+        get => _sceneMutesMic;
+        set => Set(ref _sceneMutesMic, value);
+    }
+
+    private bool? _sceneMutesDesktop;
+
+    /// <summary>TASK 35: as <see cref="SceneMutesMic"/>, for desktop/game audio.</summary>
+    public bool? SceneMutesDesktop
+    {
+        get => _sceneMutesDesktop;
+        set => Set(ref _sceneMutesDesktop, value);
+    }
+
+
     /// <summary>
     /// Asset-table id of this scene's top-strip tile artwork. The tile is a
     /// separate image from the scene's background: the preview pane keeps