Promise of the in-flight Chrome launch. Settles exactly once for the initial launch and is replaced whenever recoverBrowser runs. Awaited inside handleRun (via awaitBrowser) so a run request that arrives before Chrome is up waits the same amount of time it would have waited for the synchronous launch — but daemon start/ping/status return as soon as the socket is up. Decoupling Chrome from socket readiness is the fix for the "Daemon did not start within Ns" timeout class on slow CI.