sitesmith: pre-create section-inner/bg-section-inner/form-inner linkedNodes

The Invariant 'component type (undefined) does not exist in the resolver'
was Craft.js's toNodeTree choking on the linkedNode that <Element id="X">
auto-creates at render time inside Section / BackgroundSection /
FormContainer. The auto-created node stores its type as the Container
React component class itself, not as {resolvedName:'Container'}, so the
later type.resolvedName lookup returns undefined.

For each shell, treeToState (and apply-ai-response's buildNodeTree) now
synthesizes the linkedNode container up-front with a proper serialized
type, moves the AI's direct children into it, and reparents them. This
matches the canonical shape Craft.js writes when the user manually builds
a site, so Craft.js never has to materialize the linkedNode itself.

craft/src/utils/apply-ai-response.ts

@@ -10,6 +10,12 @@ import { SitesmithResponse, SerializedTreeNode } from '../types/sitesmith';
  * canvas but its render ignores `data.nodes`. */
 const CANVAS_TYPES = new Set(['Container']);
 
+const SHELL_INNER: Record<string, string> = {
+  Section: 'section-inner',
+  BackgroundSection: 'bg-section-inner',
+  FormContainer: 'form-inner',
+};
+
 /**
  * Flatten a SerializedTreeNode tree into a Craft.js node map ready for
  * `actions.deserialize()`.
@@ -100,6 +106,35 @@ function buildNodeTree(query: any, tree: SerializedTreeNode): NodeTree {
         craftNodes[id].data.props.columns = colCount;
       }
     }
+    // Section/BackgroundSection/FormContainer wrap their content in a single
+    // <Element id="<key>" is={Container} canvas>. Pre-create the linkedNode
+    // so Craft.js doesn't auto-create one with a malformed type field.
+    const innerKey = SHELL_INNER[node.type.resolvedName];
+    if (innerKey && craftNodes[id].data.nodes.length > 0) {
+      const innerId = `${id}__${innerKey}`;
+      const childIds: string[] = [...craftNodes[id].data.nodes];
+      craftNodes[innerId] = {
+        id: innerId,
+        data: {
+          type: { resolvedName: 'Container' },
+          props: { tag: 'div' },
+          displayName: 'Container',
+          isCanvas: true,
+          parent: id,
+          nodes: childIds,
+          linkedNodes: {},
+          hidden: false,
+          custom: {},
+        },
+        events: { selected: false, hovered: false, dragged: false },
+        rules: { canDrag: () => true, canMoveIn: () => true, canMoveOut: () => true, canDrop: () => true },
+      };
+      for (const cid of childIds) {
+        if (craftNodes[cid]) craftNodes[cid].data.parent = innerId;
+      }
+      craftNodes[id].data.nodes = [];
+      craftNodes[id].data.linkedNodes = { [innerKey]: innerId };
+    }
     return id;
   };