feat(paint-slots): concrete-plate finish + try defaults (wall=concrete, slab=wood plank 48, ceiling=soft white)

- material-library: add 'concrete-plate' KTX2 finish (512, fabric/leather-style
  pipeline) + editor-app texture mirror.
- viewer: shared resolveSlotDefaultMaterial(colour|library ref) so a kind's slot
  default can be a catalog finish, not just a flat colour.
- wall default -> library:concrete-plate (interior + exterior).
- slab default -> library:wood-woodplank48 (slab geometry resolves it via the
  shared helper).
- ceiling default -> soft white #f2eee6 (ceiling renders flat-tinted).

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

packages/core/src/material-library.ts

@@ -3860,6 +3860,44 @@ export const MATERIAL_CATALOG: MaterialCatalogItem[] = [
       },
     },
   },
+  {
+    id: 'concrete-plate',
+    label: 'Concrete Plate',
+    category: 'concrete',
+    surfaces: ['wall', 'floor'],
+    description: 'Smooth cast concrete plate',
+    previewThumbnailUrl: '/material/concrete/concrete_plate/concrete_plate_thumb.webp',
+    preset: {
+      maps: {
+        albedoMap: '/material/concrete/concrete_plate/concrete_plate_basecolor_512.ktx2',
+        normalMap: '/material/concrete/concrete_plate/concrete_plate_normal_512.ktx2',
+        roughnessMap: '/material/concrete/concrete_plate/concrete_plate_roughness_512.ktx2',
+        aoMap: '/material/concrete/concrete_plate/concrete_plate_ao_512.ktx2',
+      },
+      mapProperties: {
+        color: '#ffffff',
+        roughness: 0.8,
+        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: 'concrete-stucco',
     label: 'White Stucco',

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: '#ffffff',
-  exterior: '#ffffff',
+  interior: 'library:concrete-plate',
+  exterior: 'library:concrete-plate',
 }
 
 export type WallSurfaceMaterialSpec = {