fix(grabcraft): extract model_id fallback for external voxel JS URL

The external myRenderObject script is loaded dynamically, not via a
static <script> tag. Add fallback patterns to extract model_id or
product_id from the page HTML and construct the JS URL from that.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/grabcraft.js

@@ -323,15 +323,41 @@ function extractRenderObject(html) {
  * @returns {{ url: string, blueprintId: string } | null}
  */
 function extractExternalScriptUrl(html) {
-  const regex = /<script[^>]+src=["']((?:https?:\/\/[^"']*)?\/js\/RenderObject\/myRenderObject_(\d+)\.js)["']/i;
-  const match = html.match(regex);
-  if (!match) return null;
-
-  let url = match[1];
-  if (url.startsWith('/')) {
-    url = GRABCRAFT_BASE + url;
+  // Pattern 1: explicit <script src="...myRenderObject_XXXX.js">
+  const scriptRegex = /<script[^>]+src=["']((?:https?:\/\/[^"']*)?\/js\/RenderObject\/myRenderObject_(\d+)\.js)["']/i;
+  const scriptMatch = html.match(scriptRegex);
+  if (scriptMatch) {
+    let url = scriptMatch[1];
+    if (url.startsWith('/')) {
+      url = GRABCRAFT_BASE + url;
+    }
+    return { url, blueprintId: scriptMatch[2] };
   }
-  return { url, blueprintId: match[2] };
+
+  // Pattern 2: extract model_id from page and construct URL
+  // GrabCraft pages contain model_id in comment forms / AJAX calls
+  const modelIdRegex = /model_id\s*[:=]\s*["']?(\d+)["']?/;
+  const modelMatch = html.match(modelIdRegex);
+  if (modelMatch) {
+    const id = modelMatch[1];
+    return {
+      url: `${GRABCRAFT_BASE}/js/RenderObject/myRenderObject_${id}.js`,
+      blueprintId: id,
+    };
+  }
+
+  // Pattern 3: extract product_id
+  const productIdRegex = /product_id\s*[:=]\s*["']?(\d+)["']?/;
+  const productMatch = html.match(productIdRegex);
+  if (productMatch) {
+    const id = productMatch[1];
+    return {
+      url: `${GRABCRAFT_BASE}/js/RenderObject/myRenderObject_${id}.js`,
+      blueprintId: id,
+    };
+  }
+
+  return null;
 }
 
 /**