Enforce stale core contracts in the worker without native startup I/O

frontend/asset-sync-core.js

@@ -41,6 +41,9 @@
     const group = Object.keys(manifest.groups).find(name => manifest.groups[name].files.includes(path));
     if (!group || !previous.groups[group]?.files.includes(path) ||
         manifest.groups[group].contract !== previous.groups[group].contract) return null;
+    // A stale core must understand every group contract in the new shell.
+    if (path === "/app.js" && Object.entries(manifest.groups).some(([name, entry]) =>
+        previous.groups[name]?.contract !== entry.contract)) return null;
     return url(path, previous.files[path]);
   }