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>

packages/nodes/src/ceiling/renderer.tsx

@@ -127,7 +127,7 @@ export const CeilingRenderer = ({ node }: { node: CeilingNode }) => {
         scale={0}
         visible={false}
       />
-      {node.children.map((childId) => (
+      {(node.children ?? []).map((childId) => (
         <NodeRenderer key={childId} nodeId={childId} />
       ))}
     </mesh>

packages/nodes/src/site/renderer.tsx

@@ -139,7 +139,7 @@ export const SiteRenderer = ({ node }: { node: SiteNode }) => {
   return (
     <group ref={ref} {...handlers}>
       {/* Render children (buildings and items) */}
-      {node.children.map((childId) => (
+      {(node.children ?? []).map((childId) => (
         <NodeRenderer key={childId} nodeId={childId as AnyNodeId} />
       ))}
 

packages/nodes/src/wall/renderer.tsx

@@ -78,7 +78,7 @@ const WallRenderer = ({ node }: { node: WallNode }) => {
         {...handlers}
       />
 
-      {node.children.map((childId) => (
+      {(node.children ?? []).map((childId) => (
         <NodeRenderer key={`${node.id}:${childId}`} nodeId={childId} />
       ))}
     </mesh>