feat(paint-slots): prepared-drywall finish + fence 4 slots matching build options

- Add 'concrete-drywall' (Prepared Drywall) catalog finish + 512 KTX2/webp
  runtime assets; default walls (WALL_SLOT_DEFAULT interior+exterior) and the
  roof wall/trim band to it so roofs read continuous with walls.
- fence: replace the 2-slot panel/rail split with 4 slots that match the panel's
  build options — posts / infill (showInfill) / base (grounded only) / rail —
  each its own mesh with userData.slotId; conditional slots track the build
  state. Defaults: posts/infill/base charcoal, rail wood-finewood27.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/material-library.ts

@@ -3938,6 +3938,44 @@ export const MATERIAL_CATALOG: MaterialCatalogItem[] = [
       },
     },
   },
+  {
+    id: 'concrete-drywall',
+    label: 'Prepared Drywall',
+    category: 'concrete',
+    surfaces: ['wall', 'ceiling'],
+    description: 'Smooth prepared drywall ready for paint',
+    previewThumbnailUrl: '/material/concrete/prepared_drywall/prepared_drywall_thumb.webp',
+    preset: {
+      maps: {
+        albedoMap: '/material/concrete/prepared_drywall/prepared_drywall_basecolor_512.ktx2',
+        normalMap: '/material/concrete/prepared_drywall/prepared_drywall_normal_512.ktx2',
+        roughnessMap: '/material/concrete/prepared_drywall/prepared_drywall_roughness_512.ktx2',
+        aoMap: '/material/concrete/prepared_drywall/prepared_drywall_ao_512.ktx2',
+      },
+      mapProperties: {
+        color: '#ffffff',
+        roughness: 0.9,
+        metalness: 0,
+        repeatX: 0.5,
+        repeatY: 0.5,
+        rotation: 0,
+        wrapS: 'Repeat',
+        wrapT: 'Repeat',
+        normalScaleX: 1,
+        normalScaleY: 1,
+        emissiveIntensity: 1,
+        displacementScale: 0,
+        transparent: false,
+        flipY: false,
+        bumpScale: 1,
+        emissiveColor: '#000000',
+        aoMapIntensity: 1,
+        side: 0,
+        opacity: 1,
+        lightMapIntensity: 1,
+      },
+    },
+  },
   {
     id: 'metal-copper',
     label: 'Copper',

packages/core/src/schema/nodes/wall.ts

@@ -52,8 +52,8 @@ export type WallSurfaceSide = 'interior' | 'exterior'
 // May be a `#rrggbb` colour or a `library:<id>` ref. Textures-off still
 // collapses to the themed wall role (the escape hatch).
 export const WALL_SLOT_DEFAULT: Record<WallSurfaceSide, string> = {
-  interior: 'library:concrete-plate',
-  exterior: 'library:concrete-plate',
+  interior: 'library:concrete-drywall',
+  exterior: 'library:concrete-drywall',
 }
 
 export type WallSurfaceMaterialSpec = {