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/lib/slots.ts

@@ -0,0 +1,27 @@
+export const SLOT_MATERIAL_PREFIX = 'slot_'
+
+/** A glTF material name marks a paintable slot when it starts with `slot_` (case-insensitive). */
+export function isSlotMaterialName(name: string): boolean {
+  return name.toLowerCase().startsWith(SLOT_MATERIAL_PREFIX)
+}
+
+/**
+ * Derive the stable slot id from a glTF material name:
+ * strip the `slot_` prefix (case-insensitive), drop Blender numeric dedupe
+ * suffixes like `.001`, lowercase the remainder. Returns null when the name
+ * is not a slot material. Used by BOTH the upload scan (later) and the
+ * renderer so DB metadata and runtime meshes can never drift.
+ */
+export function deriveSlotId(materialName: string): string | null {
+  if (!isSlotMaterialName(materialName)) return null
+  let rest = materialName.slice(SLOT_MATERIAL_PREFIX.length)
+  rest = rest.replace(/\.\d+$/, '')
+  return rest.toLowerCase()
+}
+
+/** slot id -> display label: underscores to spaces, sentence case. e.g. 'bed_frame' -> 'Bed frame'. */
+export function slotLabelFromId(slotId: string): string {
+  const spaced = slotId.replace(/_/g, ' ').trim()
+  if (!spaced) return spaced
+  return spaced.charAt(0).toUpperCase() + spaced.slice(1)
+}