refactor: split LayoutStore by concern — 500-limit Phase 3, Commit E

LayoutStore.cs 1402 -> 47-line shell + 5 partials, grouped by functionality
(line count is a consequence, not the goal):

- LayoutStore.Migrations.cs: all DB schema migration (EnsureSchema, GetUserVersion,
  MigrateSourceTable/WebcamConfigTable/ToV3/SceneTable/SceneSocialBarColumn/
  SocialsTable/SocialEntryTable)
- LayoutStore.Load.cs: Load() scene/source read-back
- LayoutStore.Save.cs: Save() full scene tree write
- LayoutStore.Settings.cs: key/value settings + mic/record-folder/reusable-stream
  + license/transition/broadcast/hotkey/window-state GetSetting/UpsertSetting
- LayoutStore.Assets.cs: GetAssetBytes/UpsertAsset blob IO
- LayoutStore.cs: shell — Instance, fields, ctor, Dispose

Converted to public partial class LayoutStore : IDisposable. Pruned per-file
dead usings. Phase 3 DOWN: no production .cs exceeds 500 lines.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.

Services/LayoutStore.Assets.cs

@@ -0,0 +1,41 @@
+using System.Security.Cryptography;
+using Microsoft.Data.Sqlite;
+
+namespace ytLive.Services;
+
+public partial class LayoutStore : IDisposable
+{
+    public byte[]? GetAssetBytes(string assetId)
+    {
+        using var cmd = _connection.CreateCommand();
+        cmd.CommandText = "SELECT Data FROM Asset WHERE Id = $id";
+        cmd.Parameters.AddWithValue("$id", assetId);
+        return cmd.ExecuteScalar() as byte[];
+    }
+
+    public string UpsertAsset(byte[] data, int pixelWidth, int pixelHeight)
+    {
+        var hash = Convert.ToHexString(SHA256.HashData(data));
+        var id = Guid.NewGuid().ToString();
+
+        using var cmd = _connection.CreateCommand();
+        cmd.CommandText = """
+            INSERT INTO Asset (Id, Hash, Data, PixelWidth, PixelHeight)
+            VALUES ($id, $hash, $data, $w, $h)
+            ON CONFLICT(Hash) DO NOTHING;
+            """;
+        cmd.Parameters.AddWithValue("$id", id);
+        cmd.Parameters.AddWithValue("$hash", hash);
+        cmd.Parameters.AddWithValue("$data", data);
+        cmd.Parameters.AddWithValue("$w", pixelWidth);
+        cmd.Parameters.AddWithValue("$h", pixelHeight);
+        cmd.ExecuteNonQuery();
+
+        cmd.CommandText = "SELECT Id FROM Asset WHERE Hash = $hash;";
+        return (string)cmd.ExecuteScalar()!;
+    }
+
+    // ─── License (TASK 10 — Polar billing) ───
+
+    /// <summary>The saved license key, or null if never entered.</summary>
+}