feat(paint-slots): authored item materials + unified slot painting

Phase 1 + paint unification of the paint-slots plan.

- core: scene-material data layer (materials map mirroring collections,
  undo/partialize/setScene full-graph support), SceneMaterial schema,
  scene:/library: MaterialRef helpers + parseMaterialRef, slot id helpers
  (deriveSlotId/slotLabelFromId), slots map on ItemNode, hitObject on
  PaintResolveArgs, optional PaintCapability.commit.
- viewer: resolveMaterialRef (library:/scene: -> three material, null on
  dangling).
- nodes(item): renderer keeps authored GLB materials for slot-authored
  assets and applies per-slot overrides per-instance (never mutates the
  shared cached GLB); textures-off still collapses to furnishing role;
  non-authored items unchanged. Item paint capability + registration.
- editor: item joins the unified (nodeId, slotId) paint dispatch; item
  paint target + slot reset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/viewer/src/index.ts

@@ -68,6 +68,7 @@ export {
   MONO_PALETTE,
   PRESET_PALETTES,
   type RenderShading,
+  resolveMaterialRef,
   resolveSurfaceColor,
   WHITE_PALETTE,
 } from './lib/materials'

packages/viewer/src/lib/materials.ts

@@ -4,7 +4,10 @@ import {
   type MaterialPresetPayload,
   type MaterialProperties,
   type MaterialSchema,
+  parseMaterialRef,
   resolveMaterial,
+  type SceneMaterial,
+  type SceneMaterialId,
   type SurfaceRole,
 } from '@pascal-app/core'
 import * as THREE from 'three'
@@ -486,6 +489,24 @@ export function createMaterial(
   return threeMaterial
 }
 
+/**
+ * Resolve a MaterialRef ('library:<id>' | 'scene:<id>') to a three.js material.
+ * Returns null for an unknown / dangling ref so callers fall back to the
+ * slot's default (authored material, then themed default). Never throws.
+ */
+export function resolveMaterialRef(
+  ref: string | undefined,
+  sceneMaterials: Record<SceneMaterialId, SceneMaterial> | undefined,
+  shading: RenderShading = 'rendered',
+): THREE.Material | null {
+  const parsed = parseMaterialRef(ref)
+  if (!parsed) return null
+  if (parsed.kind === 'library') return createMaterialFromPresetRef(ref, shading)
+  const sceneMaterial = sceneMaterials?.[parsed.id as SceneMaterialId]
+  if (!sceneMaterial) return null
+  return createMaterial(sceneMaterial.material, shading)
+}
+
 export function createDefaultMaterial(
   color = '#ffffff',
   roughness = 0.9,