feat(paint-slots): unified slot defaults + paint for slab, ceiling, wall (phase 5)

Brings slab, ceiling, and wall onto the unified slot contract the shelf
established, so each declares its paintable slots with a declarative default and
(slab/ceiling) is painted through the registry capabilities.paint dispatch.

- Shared helper packages/nodes/src/shared/slot-paint.ts: a node.slots-based
  PaintCapability factory (commit/resolve/effective generic; preview injected).
  Distinct from surface-paint.ts, which writes the legacy inline node.material.
- slab: schema slots; def.geometry resolves node.slots.surface -> legacy
  material -> declared default, tags the mesh userData.slotId; slabPaint +
  capabilities.slots. Retires DEFAULT_SLAB_MATERIAL in the slab path.
- ceiling: schema slots; material builders extracted to ceiling/materials.ts
  (shared by renderer + paint preview, built BackSide so the hover preview is
  visible from below); renderer resolves the slot; ceilingPaint + slots.
- wall: WALL_SLOT_DEFAULT in core; the viewer's getMaterialsForWall renders an
  unpainted face with its declared default instead of the themed wall role;
  capabilities.slots (interior/exterior). wallPaint's inline interior/exterior
  fields are unchanged (node.slots migration is a later step).
- selection-manager + material-paint: drop slab/ceiling from the legacy
  single-surface arms (now registry-driven).

Behavior change (intended, matches the shelf precedent + the phase-5 plan):
colored-mode UNPAINTED slab/ceiling/wall surfaces now render their fixed slot
default (#e5e5e5 / #f5f5dc / #ffffff) instead of the theme role colour. The
textures-off (monochrome) role collapse is unchanged — the escape hatch.

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

packages/viewer/src/systems/wall/wall-materials.ts

@@ -2,7 +2,9 @@ import {
   getEffectiveWallSurfaceMaterial,
   getMaterialPresetByRef,
   getWallSurfaceMaterialSignature,
+  parseMaterialRef,
   resolveMaterial,
+  WALL_SLOT_DEFAULT,
   type WallNode,
   type WallSurfaceMaterialSpec,
 } from '@pascal-app/core'
@@ -12,6 +14,7 @@ import { MeshLambertNodeMaterial, MeshStandardNodeMaterial } from 'three/webgpu'
 import {
   baseMaterial,
   type ColorPreset,
+  createDefaultMaterial,
   createMaterial,
   createMaterialFromPresetRef,
   createSurfaceRoleMaterial,
@@ -88,6 +91,15 @@ function hasExplicitMaterial(spec: WallSurfaceMaterialSpec): boolean {
   return Boolean(spec.materialPreset || spec.material)
 }
 
+// Resolve a wall face's declared default — a catalog `library:` finish or a
+// flat colour — to a renderable material.
+function resolveWallSlotDefault(slotDefault: string, shading: RenderShading): Material {
+  if (parseMaterialRef(slotDefault)?.kind === 'library') {
+    return createMaterialFromPresetRef(slotDefault, shading) ?? baseMaterial(shading)
+  }
+  return createDefaultMaterial(slotDefault, 0.9, shading)
+}
+
 function getSurfaceColor(spec: WallSurfaceMaterialSpec, fallback = DEFAULT_WALL_COLOR): string {
   const preset = getMaterialPresetByRef(spec.materialPreset)
   if (preset?.mapProperties?.color) {
@@ -216,17 +228,19 @@ export function getMaterialsForWall(
   const exteriorSpec = getEffectiveWallSurfaceMaterial(wallNode, 'exterior')
   const wallRoleMaterial = createSurfaceRoleMaterial('wall', colorPreset, undefined, sceneTheme)
 
-  // Untextured surfaces take the themed wall role colour even with textures on;
-  // only surfaces with an explicit preset/material keep their texture.
+  // Colored mode: an unpainted face takes its declared slot default (parity
+  // with the retired DEFAULT_WALL_MATERIAL); only an explicit preset/material
+  // keeps a texture. Textures-off collapses every face to the themed wall role
+  // (the guaranteed escape hatch). The edge/cap slot (index 0) stays role-based.
   const visible: WallMaterialArray = textures
     ? [
         wallRoleMaterial,
         hasExplicitMaterial(interiorSpec)
           ? getSurfaceVisibleMaterial(interiorSpec, shading)
-          : wallRoleMaterial,
+          : resolveWallSlotDefault(WALL_SLOT_DEFAULT.interior, shading),
         hasExplicitMaterial(exteriorSpec)
           ? getSurfaceVisibleMaterial(exteriorSpec, shading)
-          : wallRoleMaterial,
+          : resolveWallSlotDefault(WALL_SLOT_DEFAULT.exterior, shading),
       ]
     : [wallRoleMaterial, wallRoleMaterial, wallRoleMaterial]