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

Services/LayoutStore.Migrations.cs

@@ -147,6 +147,7 @@ public partial class LayoutStore : IDisposable
         MigrateSocialEntryTable();
         if (GetUserVersion() < 3)
             MigrateToV3();
+        EnsureLayoutSnapshotSchema();
         using (var cmd = _connection.CreateCommand())
         {
             cmd.CommandText = "PRAGMA user_version = 10;";