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).

Services/LayoutStore.Migrations.cs

@@ -16,6 +16,8 @@ public partial class LayoutStore : IDisposable
                 IsHidden INTEGER NOT NULL DEFAULT 0,
                 IsChatScene INTEGER NOT NULL DEFAULT 0,
                 HasBackground INTEGER NOT NULL DEFAULT 1,
+                SceneMutesMic INTEGER NULL,
+                SceneMutesDesktop INTEGER NULL,
                 SortOrder INTEGER NOT NULL DEFAULT 0
             );
             """,
@@ -139,6 +141,7 @@ public partial class LayoutStore : IDisposable
         MigrateSceneTable();
         MigrateSceneSocialBarColumn();
         MigrateSceneThumbnailColumn();
+        MigrateSceneAudioColumns();
         MigrateSocialsTable();
         MigrateSocialEntryTable();
         if (GetUserVersion() < 3)
@@ -154,6 +157,40 @@ public partial class LayoutStore : IDisposable
     // v1 → v2: Source gains ClipShape + IsMirrored (webcam clip/mirror prefs).
     // CREATE TABLE IF NOT EXISTS doesn't touch existing tables, so pre-v2 DBs
     // get the columns here instead.
+    /// <summary>TASK 35: scene-linked audio. NULL means "leave the mix alone", so a scene is passive
+    /// until the creator says otherwise — including every layout written before this column existed.</summary>
+    private void MigrateSceneAudioColumns()
+    {
+        var sceneColumns = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        using (var cmd = _connection.CreateCommand())
+        {
+            cmd.CommandText = "PRAGMA table_info(Scene);";
+            using var reader = cmd.ExecuteReader();
+            while (reader.Read())
+                sceneColumns.Add(reader.GetString(1));
+        }
+
+        if (!sceneColumns.Contains("SceneMutesMic"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Scene ADD COLUMN SceneMutesMic INTEGER NULL;";
+            cmd.ExecuteNonQuery();
+        }
+        if (!sceneColumns.Contains("SceneMutesDesktop"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Scene ADD COLUMN SceneMutesDesktop INTEGER NULL;";
+            cmd.ExecuteNonQuery();
+        }
+
+        // BRB ships muted (creator spec), applied only where the creator has never expressed an opinion.
+        using (var cmd = _connection.CreateCommand())
+        {
+            cmd.CommandText = "UPDATE Scene SET SceneMutesMic = 1 WHERE SceneMutesMic IS NULL AND LOWER(TRIM(Name)) = 'brb';";
+            cmd.ExecuteNonQuery();
+        }
+    }
+
     private void MigrateSourceTable()
     {
         var columns = new HashSet<string>(StringComparer.OrdinalIgnoreCase);