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/core/src/index.ts

@@ -72,6 +72,12 @@ export {
   segmentsIntersect,
 } from './lib/polygon-relations'
 export { getRenderableSlabPolygon } from './lib/slab-polygon'
+export {
+  deriveSlotId,
+  isSlotMaterialName,
+  SLOT_MATERIAL_PREFIX,
+  slotLabelFromId,
+} from './lib/slots'
 export {
   type AutoCeilingPlanningContext,
   type AutoCeilingSyncPlan,
@@ -92,12 +98,18 @@ export {
   getLibraryMaterialIdFromRef,
   getMaterialPresetByRef,
   getMaterialsForCategory,
+  getSceneMaterialIdFromRef,
   LIBRARY_MATERIAL_REF_PREFIX,
   MATERIAL_CATALOG,
   MATERIAL_CATEGORIES,
   type MaterialCatalogItem,
   type MaterialCategory,
+  type MaterialRef,
+  type ParsedMaterialRef,
+  parseMaterialRef,
+  SCENE_MATERIAL_REF_PREFIX,
   toLibraryMaterialRef,
+  toSceneMaterialRef,
 } from './material-library'
 export type {
   FloorPlacedFootprint,