feat(wrap): fallback proc scan for first_child_pid_of

[noeljackson]

Summary

Add a fallback /proc scan to first_child_pid_of() for kernels without CONFIG_PROC_CHILDREN=y.

Problem

first_child_pid_of() reads /proc/<pid>/task/<pid>/children to find the supervised child process. This file only exists when the kernel is built with CONFIG_PROC_CHILDREN=y, which is not always the case (e.g., some minimal or custom kernel configs for Xen PV guests). When the file is missing, attach operations fail entirely.

Solution

1. Try the fast path first (/proc/<pid>/task/<pid>/children).
2. If that file doesn't exist or is empty, fall back to scanning /proc for numeric directories and checking each process's /proc/<pid>/status for a PPid: line matching the parent.
3. Improved error message includes the parent PID and which methods were tried.