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.Save.cs

@@ -434,10 +434,16 @@ public partial class LayoutStore : IDisposable
                 DELETE FROM Asset WHERE Id NOT IN (
                     SELECT AssetId FROM Source WHERE AssetId IS NOT NULL
                     UNION
+                    SELECT AssetId FROM DefaultSource WHERE AssetId IS NOT NULL
+                    UNION
                     SELECT ThumbnailAssetId FROM Scene WHERE ThumbnailAssetId IS NOT NULL
                     UNION
+                    SELECT ThumbnailAssetId FROM DefaultScene WHERE ThumbnailAssetId IS NOT NULL
+                    UNION
                     SELECT AlertVideoAssetId FROM Source WHERE AlertVideoAssetId IS NOT NULL
                     UNION
+                    SELECT AlertVideoAssetId FROM DefaultSource WHERE AlertVideoAssetId IS NOT NULL
+                    UNION
                     SELECT Value FROM Settings WHERE Key = 'AlertDefaultVideoAssetId'
                     UNION
                     SELECT Value FROM Settings WHERE Key = 'OnAirImageAssetId'