v1.19.0

• Use io_uring_prep_waitid for process_wait in the URing selector (Linux 6.7+), waiting for child exit directly in the ring instead of polling on a pidfd. The child is reaped via rb_process_status_wait (using WEXITED | WNOWAIT) to construct a correct Process::Status, and process_wait(-1, ...) / process_wait(0, ...) are now supported.
• Support waiting for any child or a process group (pid <= 0) on all selectors. The EPoll (pidfd_open) and KQueue (EVFILT_PROC) selectors can only watch a specific process, so these cases now fall back to a blocking wait on a dedicated thread; joining it is fiber-scheduler aware, so the reactor keeps running.