art(starting): new Starting background; the old one is archived, not lost

Creator supplied a new 2724x1536 image; resampled to the 1920x1080 master and encoded at q3
(2.9 MB → 424 KB, so the shipped asset also got smaller).

- `Assets/starting-background.jpg` — the new art, same filename, so nothing else changes
- `Assets/archive/starting-background-pre-2026-11-06.jpg` — the previous image, kept in the repo. The
  csproj lists assets explicitly, so an unreferenced file never ships.
- **Existing layouts would have kept the old art forever**: default backgrounds are hash-keyed into the
  layout DB on first seed, and the seed returns early once an AssetId exists. `HealDefaultArtVersion`
  bumps `Art.DefaultVersion` to 2 and clears the AssetId of backgrounds still using the DEFAULT (a
  creator's custom background is skipped), so the next load re-stamps the new image.

Background tests: 44 passed.

Services/LayoutStore.Settings.cs

@@ -63,6 +63,12 @@ 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");