feat(paint-slots): migrate wall + procedural kinds onto node.slots

Retire the inline material fields on every slot-model kind, moving them
onto the unified node.slots model on load so painting, edit-propagation,
and the picker behave uniformly.

Walls: slots {interior,exterior}; slot-first viewer resolution threading
sceneMaterials (content folded into the wall material hash); WallRenderer
subscribes to the scene-material palette so a scene-material edit
re-renders live; wallPaint rebuilt on createSlotPaintCapability.

Load migration generalizes legacy -> slots across slab/ceiling (surface),
fence (posts/infill/base/rail), column (shaft/base/capital/frame), shelf
(shelves/frame/back), and stair (per-role tread/side/railing). Library/
scene refs pass through; inline customs mint a deduped scene material;
legacy fields cleared. No visual change (renderers already fell back to
the legacy fields). Roof/chimney/dormer/vents intentionally stay on their
role system.

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

packages/editor/src/index.tsx

@@ -221,7 +221,6 @@ export {
   buildRoofSurfaceMaterialPatch,
   buildSingleSurfaceMaterialPatch,
   buildStairSurfaceMaterialPatch,
-  buildWallSurfaceMaterialPatch,
   getActivePaintMaterialLabel,
   hasActivePaintMaterial,
 } from './lib/material-paint'

packages/editor/src/lib/material-paint.ts

@@ -13,7 +13,6 @@ import {
   getEffectiveRoofSurfaceMaterial,
   getEffectiveSegmentSurfaceMaterial,
   getEffectiveStairSurfaceMaterial,
-  getEffectiveWallSurfaceMaterial,
   getLibraryMaterialIdFromRef,
   type MaterialSchema,
   type MaterialTarget,
@@ -26,7 +25,6 @@ import {
   type SlabNode,
   type StairNode,
   type StairSurfaceMaterialRole,
-  type WallNode,
   type WallSurfaceSide,
 } from '@pascal-app/core'
 
@@ -78,32 +76,6 @@ export function getActivePaintMaterialLabel(material: ActivePaintMaterial | null
   return getCatalogEntryForActivePaintMaterial(material)?.label ?? 'Custom'
 }
 
-export function buildWallSurfaceMaterialPatch(
-  node: WallNode,
-  targetSide: WallSurfaceSide,
-  material: MaterialSchema | undefined,
-  materialPreset: string | undefined,
-): Partial<WallNode> {
-  const nextSurfaceMaterial = { material, materialPreset }
-  const nextInterior =
-    targetSide === 'interior'
-      ? nextSurfaceMaterial
-      : getEffectiveWallSurfaceMaterial(node, 'interior')
-  const nextExterior =
-    targetSide === 'exterior'
-      ? nextSurfaceMaterial
-      : getEffectiveWallSurfaceMaterial(node, 'exterior')
-
-  return {
-    interiorMaterial: nextInterior.material,
-    interiorMaterialPreset: nextInterior.materialPreset,
-    exteriorMaterial: nextExterior.material,
-    exteriorMaterialPreset: nextExterior.materialPreset,
-    material: undefined,
-    materialPreset: undefined,
-  }
-}
-
 export function buildRoofSurfaceMaterialPatch(
   node: RoofNode,
   targetRole: RoofSurfaceMaterialRole,