Opens the three surfaces as three separate same-origin documents (iframes, one per surface — a genuine multi-document, multi-BroadcastChannel-endpoint test, not a simulation) and drives the required proof points listed in the Step 6.2 brief. Every wait below is condition-based (polls a real signal — attachment state, DOM content, or stateRevision — until it is true or a timeout elapses); nothing is timed by a fixed sleep standing in for an actual event. Click "Run checks" once all three frames have loaded.