fix(nodes): coalesce node.children to [] in container renderers (#333)

Guards building/ceiling/site/wall renderers with `(node.children ?? [])` so a node whose `children` array is missing (legacy/unparsed scene data) no longer crashes the renderer with "Cannot read properties of undefined (reading 'map')" (EDITOR-C0). Matches the existing guard in roof/renderer.tsx and the Array.isArray check in the parametric renderer.

Note: the schema declares `children: z.array(...).default([])`, so this can only be hit by data that bypasses Zod normalization on load. This is a defense-in-depth crash-stopper; the deeper fix is to normalize/parse legacy nodes in migrateNodes (use-scene.ts) so missing arrays are repaired before render — tracked as a follow-up.

packages/nodes/src/building/renderer.tsx

@@ -18,7 +18,7 @@ export const BuildingRenderer = ({ node }: { node: BuildingNode }) => {
       rotation={[node.rotation[0], node.rotation[1], node.rotation[2]]}
       {...handlers}
     >
-      {node.children.map((childId) => (
+      {(node.children ?? []).map((childId) => (
         <NodeRenderer key={childId} nodeId={childId} />
       ))}
     </group>