Function set_parent_death_signal 

pub fn set_parent_death_signal(parent_pid: pid_t) -> Result<()>

Ensure the child receives SIGTERM when the original parent dies.

This should run in pre_exec and uses parent_pid captured before spawn to avoid a race where the parent exits between fork and exec.

Safety

This function uses unsafe libc calls and should only be called from a pre_exec hook.