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

ytLive.Tests/LayoutStoreSnapshotTests.cs

@@ -0,0 +1,64 @@
+using System;
+using System.IO;
+using System.Linq;
+using Microsoft.Data.Sqlite;
+using Xunit;
+using ytLive.Models;
+using ytLive.Services;
+
+namespace ytLive.Tests;
+
+public sealed class LayoutStoreSnapshotTests
+{
+    [Fact]
+    public void SameBuild_OpensCurrentLayout_WithoutChangingDefaultSnapshot()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"ytLlive-layout-snapshot-{Guid.NewGuid():N}.db");
+        const string buildId = "build-a";
+        try
+        {
+            using (var store = new LayoutStore(path))
+            {
+                var factory = MakeScene("Factory title");
+                store.Save(new[] { factory }, null, null);
+                store.PrepareLayoutSnapshots(buildId);
+
+                store.Save(new[] { MakeScene("Creator edit") }, null, null);
+            }
+
+            using (var reopened = new LayoutStore(path))
+            {
+                reopened.PrepareLayoutSnapshots(buildId);
+                var current = Assert.Single(reopened.Load());
+                Assert.Equal("Creator edit", Assert.Single(current.Elements.OfType<Source>()).Name);
+
+                using var connection = GetConnection(path);
+                using var command = connection.CreateCommand();
+                command.CommandText = "SELECT Value FROM LayoutSnapshotState WHERE Key = 'CurrentBuildId';";
+                Assert.Equal(buildId, command.ExecuteScalar());
+
+                command.CommandText = "SELECT Name FROM DefaultSource;";
+                Assert.Equal("Factory title", command.ExecuteScalar());
+            }
+        }
+        finally
+        {
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(path); } catch { /* best-effort cleanup */ }
+        }
+    }
+
+    private static Scene MakeScene(string sourceName)
+        => new()
+        {
+            Name = SceneCatalog.Starting,
+            Elements = { new Source { Name = sourceName, Type = SourceType.TextOverlay } },
+        };
+
+    private static SqliteConnection GetConnection(string path)
+    {
+        var connection = new SqliteConnection($"Data Source={path}");
+        connection.Open();
+        return connection;
+    }
+}