feat(37): reset one scene to its factory snapshot

The per-scene reset restores that scene's settings, layers, webcam placement,
and background-path settings while leaving other scenes' current edits alone.
A focused integration test covers the restore/isolation behavior.
611 tests green, clean build with 0 warnings.

Reference: OBS scene collections store scenes and sources as a saved set:
https://obsproject.com/kb/scene-collections

ytLive.Tests/LayoutStoreSnapshotTests.cs

@@ -80,10 +80,57 @@ public sealed class LayoutStoreSnapshotTests
         }
     }
 
+    [Fact]
+    public void ResetOneScene_RestoresItsFactoryLayers_AndKeepsOtherSceneEdits()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"ytLlive-layout-snapshot-{Guid.NewGuid():N}.db");
+        var starting = MakeScene(SceneCatalog.Starting, "Factory starting");
+        var brb = MakeScene(SceneCatalog.Brb, "Factory BRB");
+        try
+        {
+            using (var store = new LayoutStore(path))
+            {
+                store.Save(new[] { starting, brb }, null, null);
+                store.PrepareLayoutSnapshots("build-a");
+                store.Save(new[]
+                {
+                    MakeScene(starting.Id, starting.Name, "Edited starting"),
+                    MakeScene(brb.Id, brb.Name, "Edited BRB"),
+                }, null, null);
+            }
+
+            using (var reopened = new LayoutStore(path))
+            {
+                reopened.PrepareLayoutSnapshots("build-a");
+                Assert.True(reopened.RestoreDefaultScene(starting.Id));
+
+                var scenes = reopened.Load();
+                Assert.Equal("Factory starting", Assert.Single(scenes.Single(s => s.Id == starting.Id).Elements.OfType<Source>()).Name);
+                Assert.Equal("Edited BRB", Assert.Single(scenes.Single(s => s.Id == brb.Id).Elements.OfType<Source>()).Name);
+            }
+        }
+        finally
+        {
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(path); } catch { /* best-effort cleanup */ }
+        }
+    }
+
     private static Scene MakeScene(string sourceName)
+        => MakeScene(SceneCatalog.Starting, sourceName);
+
+    private static Scene MakeScene(string sceneName, string sourceName)
         => new()
         {
-            Name = SceneCatalog.Starting,
+            Name = sceneName,
+            Elements = { new Source { Name = sourceName, Type = SourceType.TextOverlay } },
+        };
+
+    private static Scene MakeScene(string id, string sceneName, string sourceName)
+        => new()
+        {
+            Id = id,
+            Name = sceneName,
             Elements = { new Source { Name = sourceName, Type = SourceType.TextOverlay } },
         };