feat(canvas): send the last referenced canvas node as a generation anchor

Resolve the most recent canvas-node reference from the prompt's reference
images (by node id or matching asset URL) and append it as a hidden
"reference anchor node:" settings directive on agent submissions, so the
server can place generated images beside the source. Keep the persisted
user-visible copy unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/CanvasWorkspace/canvas/nodeReferenceImages.ts

@@ -1,4 +1,4 @@
-import type { CanvasNode } from "@/domain/design";
+import type { AgentContent, CanvasNode } from "@/domain/design";
 import type { UploadedReferenceImage } from "@/ui/components/PromptComposer";
 
 const canvasNodeReferencePrefix = "canvas-node:";
@@ -21,6 +21,31 @@ export function isCanvasNodeReferenceImage(reference: UploadedReferenceImage) {
   return reference.id.startsWith(canvasNodeReferencePrefix);
 }
 
+export function lastCanvasNodeIdForReferences(references: Array<Pick<UploadedReferenceImage, "id" | "publicUrl">>, nodes: Array<Pick<CanvasNode, "id" | "content">>) {
+  for (let index = references.length - 1; index >= 0; index -= 1) {
+    const reference = references[index];
+    const directNodeId = reference.id.startsWith(canvasNodeReferencePrefix) ? reference.id.slice(canvasNodeReferencePrefix.length) : "";
+    if (directNodeId && nodes.some((node) => node.id === directNodeId)) return directNodeId;
+    const referenceKeys = assetReferenceKeys(reference.publicUrl);
+    const matchedNode = [...nodes].reverse().find((node) => Array.from(assetReferenceKeys(node.content)).some((key) => referenceKeys.has(key)));
+    if (matchedNode) return matchedNode.id;
+  }
+  return "";
+}
+
+export function withLastCanvasReferenceAnchor(contents: AgentContent[], references: Array<Pick<UploadedReferenceImage, "id" | "publicUrl">>, nodes: Array<Pick<CanvasNode, "id" | "content">>): AgentContent[] {
+  const anchorNodeId = lastCanvasNodeIdForReferences(references, nodes);
+  if (!anchorNodeId) return contents;
+  return [
+    ...contents,
+    {
+      type: "text",
+      text: `Reference anchor node: ${anchorNodeId}`,
+      textSource: "settings"
+    }
+  ];
+}
+
 export function isAssetReferencedByCanvas(publicUrl: string, nodes: CanvasNode[]) {
   const targetKeys = assetReferenceKeys(publicUrl);
   if (targetKeys.size === 0) return false;