Reproduces the user's reported real-world shape end to end with a real
busybox container ("nohup sleep 137 & exit"), verifying via the host's own
/proc that the backgrounded process is actually gone afterward -- checked
by cmdline substring, not pid, since a pid seen inside an isolated pid
namespace doesn't correspond to the same-numbered host pid. A bounded 2s
poll guards against the pid namespace's own kernel collapse-on-pid-1-exit
timing (which already covers this case for free on this dev machine).
Complements test_session_cleanup.cpp's existing [integration][root] test,
which exercises kill_via_cgroup() directly -- this one instead proves the
outward, visible contract holds through the real -r/--run path.
ceamac
a22024fcf8