feat(37): keep a default layout snapshot beside the current layout

The matching-build persistence slice is in place: existing layouts are captured
once as defaults, first-run factory scenes are saved and captured before the
creator can edit, and same-build startup continues to load current rows. Default
asset references survive current-layout saves.

One integration test covers the current-layout roundtrip and immutable default.
609 tests green, clean build with 0 warnings.

Reference scan: OBS stores scene collections separately from output profiles;
this split keeps layout state independent in the same SQLite store:
https://obsproject.com/kb/scene-collections

ViewModels/MainViewModel.cs

@@ -514,9 +514,11 @@ public partial class MainViewModel : ViewModelBase
     private void LoadLayout()
     {
         _isLoading = true;
+        var seededFactoryLayout = false;
         try
         {
             AppLog.Write("LoadLayout begin");
+            _layoutStore.PrepareLayoutSnapshots(Helpers.BuildStamp.Id);
             var scenes = _layoutStore.Load();
             Scenes.Clear();
             foreach (var scene in scenes)
@@ -526,6 +528,7 @@ public partial class MainViewModel : ViewModelBase
             {
                 foreach (var name in SceneCatalog.All)
                     AddScene(name);
+                seededFactoryLayout = true;
             }
             ReorderScenesToCanonical();
             HealBackgrounds();
@@ -582,6 +585,12 @@ public partial class MainViewModel : ViewModelBase
         LoadClipSettings();
         BroadcastForm.Load();
 
+        if (seededFactoryLayout)
+        {
+            SaveLayoutNow();
+            _layoutStore.CaptureDefaultsIfMissing();
+        }
+
         ScheduleSave();
         AppLog.Write("LoadLayout end");
     }