Cache the page-owned app response before starting the worker

perf/measurement.mjs

@@ -27,3 +27,8 @@ export function assertWarmAssets(requests) {
  const assets=requests.filter(r=>r.bytes>0&&/\.(?:js|css|woff2?|png|svg)(?:[?#]|$)/.test(r.url)&&!new URL(r.url,'http://localhost').pathname.endsWith('/sw.js'));
  assert.deepEqual(assets,[],'warm reload does not transfer unfinished shell assets');
 }
+
+export function assertColdAssets(requests) {
+ const transferred=requests.filter(r=>r.bytes>0&&new URL(r.url,'http://localhost').pathname==='/app.js');
+ assert.equal(transferred.length,1,'cold app.js transfers its body exactly once across page and worker');
+}