feat: /proc per-process directories (/proc/[pid]/status, maps)

feat: /proc per-process directories (/proc/[pid]/status, maps)

- /proc/[pid]/status: shows Pid, PPid, Pgrp, Session, State, signals, heap
- /proc/[pid]/maps: shows heap range and mmap regions
- /proc root readdir now lists numeric PID entries alongside self/uptime/meminfo
- /proc root finddir resolves numeric names to per-PID directory nodes
- Uses small static pool of fs_node_t (8 slots) for dynamic PID nodes
- proc_find_pid helper iterates ready queue to find process by PID
- cppcheck clean, 19/19 smoke tests pass