fix(art): re-seed a bundled background when the stored bytes differ

The creator's profile still showed the previous Starting art after the replacement, with
`Art.DefaultVersion = 4` recorded — so the version gate had already declared itself done. It
compared a constant against the embedded resource, and those two were compiled at different
moments (version 4 landed with the OLD image embedded; the corrected image reached the assembly
one build later). A version bump can always be raced that way.

The seed now compares BYTES: `LayoutStore.AssetMatches` hashes the bundled resource and checks the
row the background already points at. Same bytes → leave it untouched (so this is idempotent and
never churns the database). Different bytes → re-stamp. The version constant and its setting are
gone, because a heal that only runs once can silently skip the one launch it was needed for.

`SceneThumbnailTests` planted its own art on a background still flagged as default art — in the
real app that state only ever means stale bundled art, because Browse… sets the path and clears
the flag. The fixture now models a creator's own picture.

Tests: 572 green, 0 warnings. Verified the profile's Starting asset hash (167CDD3B…, the
two-category render) differs from the shipped `starting-background.jpg` (5FC6DD22…), so the next
launch re-seeds it.

Services/LayoutStore.Assets.cs

@@ -35,6 +35,20 @@ public partial class LayoutStore : IDisposable
         return (string)cmd.ExecuteScalar()!;
     }
 
+    /// <summary>True when the asset already holds exactly these bytes. Used to decide whether a
+    /// bundled image needs re-seeding: comparing bytes cannot go stale the way a version constant
+    /// can (the constant and the embedded resource were built at different moments twice).</summary>
+    public bool AssetMatches(string assetId, byte[] data)
+    {
+        if (string.IsNullOrWhiteSpace(assetId)) return false;
+        var hash = Convert.ToHexString(SHA256.HashData(data));
+        using var cmd = _connection.CreateCommand();
+        cmd.CommandText = "SELECT 1 FROM Asset WHERE Id = $id AND Hash = $hash;";
+        cmd.Parameters.AddWithValue("$id", assetId);
+        cmd.Parameters.AddWithValue("$hash", hash);
+        return cmd.ExecuteScalar() != null;
+    }
+
     // ─── License (TASK 10 — Polar billing) ───
 
     /// <summary>The saved license key, or null if never entered.</summary>