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.

ViewModels/MainViewModel.Background.cs

@@ -427,9 +427,30 @@ public partial class MainViewModel
     /// <summary>Heals every scene to the locked Background model after a layout
     /// load: keeps the correct flavor per scene at index 0 named "Background",
     /// drops duplicates/stale rows, re-seeds missing ones.</summary>
+    /// <summary>Bump when a bundled background image is replaced. Layouts stamped with an older version
+    /// re-seed their DEFAULT art on next load; a creator's own custom background is never touched.</summary>
+    private const int CurrentDefaultArtVersion = 2;
+
+    /// <summary>Re-stamp the shipped art for scenes still using the default. Without this, a replaced
+    /// <c>starting-background.jpg</c> reaches new installs only: the asset is hash-keyed into the layout
+    /// database on first seed, so an existing profile keeps showing the old image forever.</summary>
+    private void HealDefaultArtVersion()
+    {
+        if (_layoutStore.LoadDefaultArtVersion() >= CurrentDefaultArtVersion) return;
+
+        foreach (var background in AllBackgrounds())
+        {
+            if (!background.UseDefaultBackground) continue;   // the creator chose their own art
+            background.AssetId = null;                        // seed re-stamps it below
+        }
+
+        _layoutStore.SaveDefaultArtVersion(CurrentDefaultArtVersion);
+    }
+
     private void HealBackgrounds()
     {
         NormalizeBackgrounds(Scenes);
+        HealDefaultArtVersion();
         foreach (var scene in Scenes)
         {
             _sceneGraph.InvalidateBake(scene);