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/schema/index.ts

@@ -158,6 +158,7 @@ export {
 } from './nodes/wall'
 export { WindowNode, WindowType } from './nodes/window'
 export { ZoneNode } from './nodes/zone'
+export { generateSceneMaterialId, SceneMaterial, type SceneMaterialId } from './scene-material'
 export type { AnyNodeId, AnyNodeType } from './types'
 // Union types
 export { AnyNode } from './types'

packages/core/src/schema/nodes/item.ts

@@ -146,6 +146,11 @@ export const ItemNode = BaseNode.extend({
   // Denormalized references to collections this node belongs to
   collectionIds: z.array(z.custom<CollectionId>()).optional(),
 
+  // Per-slot material overrides. Key = slot id (see deriveSlotId), value = a
+  // MaterialRef string ('library:<id>' or 'scene:<id>'). Absent = authored /
+  // registry default. A dangling ref renders the default (never blocks).
+  slots: z.record(z.string(), z.string()).optional(),
+
   asset: assetSchema,
 }).describe(dedent`Item node - used to represent a item in the building
   - position: position in level coordinate system (or parent coordinate system if attached)

packages/core/src/schema/scene-material.ts

@@ -0,0 +1,13 @@
+import { z } from 'zod'
+import { generateId } from './base'
+import { MaterialSchema } from './material'
+
+export type SceneMaterialId = `mat_${string}`
+export const generateSceneMaterialId = (): SceneMaterialId => generateId('mat')
+
+export const SceneMaterial = z.object({
+  id: z.string(),
+  name: z.string(),
+  material: MaterialSchema,
+})
+export type SceneMaterial = z.infer<typeof SceneMaterial>