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.

ytLive.Tests/SceneThumbnailTests.cs

@@ -263,6 +263,11 @@ public sealed class SceneThumbnailTests
                 var id = LayoutStore.Instance!.UpsertAsset(bytes, 320, 180);
                 Assert.NotNull(id);
                 bg!.AssetId = id;
+                // The creator's own picture, not the shipped art: a default-art background pointing at
+                // other bytes is re-seeded by StampDefaultBackgroundAsset (2026-11-06), and this fixture
+                // needs its planted art to stay put. Browse… always sets the path alongside the asset.
+                bg.UseDefaultBackground = false;
+                bg.CustomBackgroundPath = Path.Combine(Path.GetTempPath(), $"ytLive-missing-{i}.jpg");
                 backgroundIds[scene.Name] = id;
             }
             builder.SaveLayoutNow();