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.Settings.cs

@@ -63,12 +63,6 @@ public partial class LayoutStore : IDisposable
 
     public void SaveShowMonetaryMetrics(bool show) => UpsertSetting("Metrics.ShowMonetary", show ? "1" : "0");
 
-    /// <summary>Which vintage of the shipped default scene art this layout was stamped with. Bumped when
-    /// a bundled background is replaced, so existing layouts re-seed instead of keeping the old art.</summary>
-    public int LoadDefaultArtVersion() => int.TryParse(GetSetting("Art.DefaultVersion"), out var v) ? v : 1;
-
-    public void SaveDefaultArtVersion(int version) => UpsertSetting("Art.DefaultVersion", version.ToString());
-
     public bool LoadAutoChromaScreen() => GetSetting("Video.AutoChromaScreen") == "1";
 
     public void SaveAutoChromaScreen(bool enabled) => UpsertSetting("Video.AutoChromaScreen", enabled ? "1" : "0");