Function inherited_fds 

fn inherited_fds() -> Vec<RawFd> 

Every open descriptor above stderr. Split out from close_inherited_fds so it can be tested without closing the test runner’s own descriptors.

This is Linux only: it enumerates /proc/self/fd and would report nothing at all where that is unavailable, silently leaking the inherited descriptors. conmon-rs already requires Linux elsewhere (prctl, epoll, journald), so the assertion below just makes a port fail here loudly rather than lose the behaviour quietly. A log line would not work, close_inherited_fds runs well before the logging got set up.