Function await_running_instance 

pub(crate) fn await_running_instance<T>(
    try_claim: impl FnMut() -> Option<T>,
    try_forward: impl FnMut() -> Result<(), ForwardError>,
    first: ForwardError,
) -> Handoff<T>

A relaunch whose forward failed with first: the running instance is starting (no port file yet) or quitting (its host is gone; a clean exit deletes the port file). Until QUITTING_INSTANCE_WAIT passes, take the socket/pipe as soon as the old launcher lets go of it, or forward as soon as the host answers. Shared by both supervisors.