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>
