docs: update FULL_POSIX_AUDIT.md — all Tier 1-5 items resolved

docs: update FULL_POSIX_AUDIT.md — all Tier 1-5 items resolved

- umount2 (syscall 138) marked implemented
- wait4 (syscall 139) marked implemented
- All 16 new ulibc headers marked implemented
- All missing functions in stdio/stdlib/string/unistd/termios marked resolved
- pthread sync primitives (mutex/cond/rwlock/key/once/barrier) marked implemented
- /etc/passwd and /etc/group parsing marked implemented
- ENOTSOCK/ENETUNREACH errno codes marked implemented
- Syscall count updated: 139 (138 entries), ~99% POSIX
- All Bash blockers resolved (getpwnam etc.)
- Busybox blockers reduced (getpwent/syslog now done)