editor: gate Load Build behind a verification dialog

Loading JSON previously called setScene blindly and crashed when
the file held schema-invalid nodes (e.g. items missing `asset`).
The new dialog parses the file, runs validateBuildJson in core,
and surfaces structure counts (site/building/levels/walls/doors/
windows/items/slabs/ceilings/zones/scans), floor area, and a
per-node Schema Details list grouped by type. Import is blocked
when any hard error exists.

Two schema bugs the validator surfaced are fixed here too:

- SiteNode.children is now an id array like every other node
  (was a discriminatedUnion of full objects; three readers carried
  a string-or-object ternary that's now dropped). migrateNodes
  flattens legacy nested-object children on load. Default-scene
  seed and photo-to-scene MCP builder updated to pass `building.id`.
- LevelNode.children now includes shelf — the editor allowed it
  but the schema didn't. The schema-vs-registry-as-source-of-truth
  discussion is captured in plans/editor-node-registry.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -8,6 +8,7 @@ import { GuideNode } from './guide'
 import { ItemNode } from './item'
 import { RoofNode } from './roof'
 import { ScanNode } from './scan'
+import { ShelfNode } from './shelf'
 import { SlabNode } from './slab'
 import { SpawnNode } from './spawn'
 import { StairNode } from './stair'
@@ -32,6 +33,7 @@ export const LevelNode = BaseNode.extend({
         ScanNode.shape.id,
         GuideNode.shape.id,
         SpawnNode.shape.id,
+        ShelfNode.shape.id,
       ]),
     )
     .default([]),

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

@@ -3,8 +3,6 @@
 import dedent from 'dedent'
 import { z } from 'zod'
 import { BaseNode, nodeType, objectId } from '../base'
-import { BuildingNode } from './building'
-import { ItemNode } from './item'
 
 // 2D Polygon
 const PropertyLineData = z.object({
@@ -33,14 +31,12 @@ export const SiteNode = BaseNode.extend({
     ],
   }),
   // terrain: TerrainData,
-  children: z
-    .array(z.discriminatedUnion('type', [BuildingNode, ItemNode]))
-    .default([BuildingNode.parse({})]),
+  children: z.array(z.string()).default([]),
 }).describe(
   dedent`
   Site node - used to represent a site
   - polygon: polygon data
-  - children: array of building and item nodes
+  - children: array of child node ids (buildings, items)
   `,
 )