feat: multiple PTY pairs (up to 8 dynamic /dev/pts/N)

feat: multiple PTY pairs (up to 8 dynamic /dev/pts/N)

Refactored PTY subsystem from single global pair to array of up to 8 pairs:
- New pty_pair struct with per-pair buffers, waitqueues, session/pgrp
- pty_alloc_pair() allocates new pairs dynamically
- Inode encoding: masters=100+N, slaves=200+N
- pty_get_master_node()/pty_get_slave_node() return per-pair fs_node_t
- All _idx() variants for indexed pair access
- Old single-pair API preserved as wrappers around pair 0
- devfs /dev/pts/ now lists all active pairs dynamically
- syscall.c poll/nonblock/ioctl updated to use pty_is_master_ino/pty_is_slave_ino
- cppcheck clean, 19/19 smoke tests pass