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/nodes/src/item/definition.ts

@@ -7,6 +7,7 @@ import {
 } from '@pascal-app/core'
 import { buildItemFloorplan } from './floorplan'
 import { itemFloorplanMoveTarget } from './floorplan-move'
+import { itemPaint } from './paint'
 import { itemParametrics } from './parametrics'
 import { ItemNode } from './schema'
 
@@ -199,6 +200,7 @@ export const itemDefinition: NodeDefinition<typeof ItemNode> = {
     selectable: { hitVolume: 'bbox' },
     duplicable: true,
     deletable: true,
+    paint: itemPaint,
     // Items participate in compositions — e.g. "table-with-plants",
     // "shelf-with-books-on-top" — so they're presettable in their own
     // right (and as descendants of presettable parents). The GLB-kind