fix: update mcp and nodes for SiteNode.children string[] schema change

Fixes breakage introduced by #320 which changed `SiteNode.children` from
embedded `BuildingNode | ItemNode` objects to flat `string[]` IDs.

- `packages/mcp/src/lib/rehydrate-site-children.ts`: replace now-obsolete
  re-embedding logic with a no-op passthrough (call-site compatible)
- `packages/mcp/src/tools/variants/generate-variants.ts`: drop the inline
  copy of the same function and its call
- `packages/nodes/src/site/renderer.tsx`: cast `childId as AnyNodeId` since
  `SiteNode.children` is now `string[]`, not `AnyNodeId[]`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/nodes/src/site/renderer.tsx

@@ -1,6 +1,6 @@
 'use client'
 
-import { type SiteNode, type SlabNode, useRegistry, useScene } from '@pascal-app/core'
+import { type AnyNodeId, type SiteNode, type SlabNode, useRegistry, useScene } from '@pascal-app/core'
 import { NodeRenderer, unionPolygons, useNodeEvents, useViewer } from '@pascal-app/viewer'
 import { useMemo, useRef } from 'react'
 import { BufferGeometry, Float32BufferAttribute, type Group, Path, Shape } from 'three'
@@ -117,7 +117,7 @@ export const SiteRenderer = ({ node }: { node: SiteNode }) => {
     <group ref={ref} {...handlers}>
       {/* Render children (buildings and items) */}
       {node.children.map((childId) => (
-        <NodeRenderer key={childId} nodeId={childId} />
+        <NodeRenderer key={childId} nodeId={childId as AnyNodeId} />
       ))}
 
       {/* Ground fill: site polygon with slab holes, occludes below-grade geometry */}