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).

ytLive.Tests/SceneAudioTests.cs

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+using System;
+using System.IO;
+using System.Linq;
+using Microsoft.Data.Sqlite;
+using Xunit;
+using ytLive.Models;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// TASK 35 — the scene remembers the room.
+///
+/// <para>The rule the test pins, in the creator's words: a scene's audio state is applied <b>once when
+/// it takes the program output</b>. `null` means "leave the mix alone", so staging a scene offline, or
+/// switching to a scene with no opinion, must never undo a manual mute.</para>
+/// </summary>
+[Collection("RealApp")]
+public sealed class SceneAudioTests
+{
+    private readonly RealAppHost _app;
+
+    public SceneAudioTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void EnteringALiveScene_AppliesItsAudioState_OnceAndOnlyLive() => _app.Run(() =>
+    {
+        var tempDb = Path.Combine(Path.GetTempPath(), $"ytLlive-sceneaudio-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
+        try
+        {
+            var vm = new MainViewModel();
+            var brb = vm.Scenes.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Brb));
+            var chat = vm.Scenes.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Chat));
+
+            // Ships muted (the creator's default), while every other scene is passive.
+            Assert.True(brb.SceneMutesMic);
+            Assert.Null(chat.SceneMutesMic);
+            Assert.Null(brb.SceneMutesDesktop);
+
+            vm.MicVolume = 0.8;
+            Assert.False(vm.MicMuted);
+
+            // Staging offline: no program output, so no audio edit. The creator is auditioning.
+            vm.TransitionToScene(brb);
+            Assert.False(vm.MicMuted);
+
+            // Live: entering BRB mutes the mic.
+            vm.StreamStatus = StreamStatus.Streaming;
+            vm.TransitionToScene(vm.Scenes.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Live)));
+            vm.TransitionToScene(brb);
+            Assert.True(vm.MicMuted);
+
+            // The creator is boss: a manual unmute survives a scene with no opinion.
+            vm.SetMicMuted(false);
+            Assert.False(vm.MicMuted);
+            vm.TransitionToScene(chat);
+            Assert.False(vm.MicMuted);
+
+            // ...but a scene that DOES have an opinion wins on its next entry.
+            chat.SceneMutesMic = true;
+            vm.TransitionToScene(brb);
+            vm.TransitionToScene(chat);
+            Assert.True(vm.MicMuted);
+
+            // Desktop audio was never told anything, so it was never touched.
+            Assert.False(vm.GameMuted);
+
+            // Roundtrip: the three states survive a save + reload, null included.
+            vm.SaveLayoutNow();
+            SqliteConnection.ClearAllPools();
+            using var store = new LayoutStore(tempDb);
+            var reloaded = store.Load();
+            var reloadedBrb = reloaded.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Brb));
+            var reloadedChat = reloaded.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Chat));
+            var reloadedLive = reloaded.Single(s => SceneCatalog.Is(s.Name, SceneCatalog.Live));
+            Assert.True(reloadedBrb.SceneMutesMic);
+            Assert.True(reloadedChat.SceneMutesMic);
+            Assert.Null(reloadedLive.SceneMutesMic);
+        }
+        finally
+        {
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(tempDb); } catch { /* best effort */ }
+        }
+    });
+}