feat: add reusable custom decor images

Issue: #51
User-Visible: yes

scripts/config-field-registry.mjs

@@ -411,6 +411,20 @@ export const CONFIG_FIELD_REGISTRY = Object.freeze([
     migration: 'none',
     compatibility: 'unknown symbols render as generic box; assets ship with the bundle',
   },
+  {
+    id: "spaces[].decor[]<kind='image'>",
+    selector: { path: ['spaces', '*', 'decor', '*', 'kind'], equals: 'image' },
+    storage: 'House Plan server config + content-addressed houseplan/assets store',
+    type: 'decor item variant: asset_id + box (x, y, w, h, angle, opacity, flip_h, flip_v)',
+    default: 'n/a',
+    level: 'space',
+    ui: 'Background editor Image palette (#51)',
+    runtime: 'full/static decor projection through verified signed content',
+    introduced: 'v1.71.0',
+    status: 'current',
+    migration: 'none — additive record; file bytes are never embedded in config',
+    compatibility: 'capability-gated writes; missing bytes keep an editor repair placeholder',
+  },
   {
     id: "spaces[].openings[].host=partition",
     selector: { path: ['spaces', '*', 'openings', '*', 'host', 'kind'], equals: 'partition' },

scripts/mutation-gate.mjs

@@ -1428,7 +1428,7 @@ const MUTANT_DEFINITIONS = [
       + 'decodes unwarned and kills the tablet tab (#39 AC1)',
     patches: [{
       file: 'src/backdrop-pick.ts',
-      find: "  if (probe.kind === 'safe') return { kind: 'pass', ext };",
+      find: "  if (probe.kind === 'safe' && file.size <= guardAboveBytes) return { kind: 'pass', ext };",
       replace: "  return { kind: 'pass', ext };",
     }],
   },
@@ -3244,13 +3244,16 @@ const MUTANT_DEFINITIONS = [
       replace: '  spaceModels, roomCenter, defaultPositions, markerPos, labelPos, spaceFrame, iconCqw, NORM_W,',
     }, {
       file: 'src/space-render.ts',
-      find: '        ${passageGlowTunnels}\n        ${glowPools}\n        ${wallUnion',
+      find: '        ${passageGlowTunnels}\n'
+        + '        <g class="decorlayer" pointer-events="none">${decorImages}</g>\n'
+        + '        ${glowPools}\n        ${wallUnion',
       replace: '        ${passageGlowTunnels}\n'
         + '        ${!space.bg && !disp.showNames ? svg`<g class="room-svg-labels" pointer-events="none">${space.rooms.map((room) => {\n'
         + '          const center = roomCenter(room);\n'
         + '          return svg`<text class="rlabel" data-hp="room-label" data-id=${room.id || nothing}\n'
         + '            data-area=${room.area || nothing} x=${center[0]} y=${center[1]}>${room.name}</text>`;\n'
         + '        })}</g>` : nothing}\n'
+        + '        <g class="decorlayer" pointer-events="none">${decorImages}</g>\n'
         + '        ${glowPools}\n'
         + '        ${wallUnion',
     }],