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/viewer/src/index.ts

@@ -69,6 +69,7 @@ export {
   PRESET_PALETTES,
   type RenderShading,
   resolveMaterialRef,
+  resolveSlotDefaultMaterial,
   resolveSurfaceColor,
   WHITE_PALETTE,
 } from './lib/materials'

packages/viewer/src/lib/materials.ts

@@ -516,6 +516,26 @@ export function resolveMaterialRef(
   return createMaterial(sceneMaterial.material, shading)
 }
 
+/**
+ * Resolve a node kind's declared slot default — either a catalog `library:<id>`
+ * finish or a flat `#rrggbb` colour — to a renderable material. Shared by the
+ * procedural kinds whose colored-mode unpainted appearance comes from a
+ * declarative default (slab, wall).
+ */
+export function resolveSlotDefaultMaterial(
+  slotDefault: string,
+  shading: RenderShading = 'rendered',
+  roughness = 0.9,
+): THREE.Material {
+  if (parseMaterialRef(slotDefault)?.kind === 'library') {
+    return (
+      createMaterialFromPresetRef(slotDefault, shading) ??
+      createDefaultMaterial('#ffffff', roughness, shading)
+    )
+  }
+  return createDefaultMaterial(slotDefault, roughness, shading)
+}
+
 export function createDefaultMaterial(
   color = '#ffffff',
   roughness = 0.9,