feat: implement POSIX Tiers 1-5 — complete ulibc headers, syscalls, and sync primitives

feat: implement POSIX Tiers 1-5 — complete ulibc headers, syscalls, and sync primitives

Tier 1: Add ENOTSOCK/ENETUNREACH errno codes (kernel + ulibc)

Tier 2: 13 new ulibc wrapper headers + source files:
  - sys/socket.h, netinet/in.h, arpa/inet.h, netdb.h, sys/un.h
  - sys/epoll.h, sys/inotify.h, dlfcn.h, spawn.h
  - semaphore.h, mqueue.h, aio.h, sys/shm.h
  - All with syscall wrappers matching kernel syscall numbers

Tier 3: Missing functions in existing headers:
  - termios: cfmakeraw, cfget/setispeed, tcdrain, tcflush, tcgetpgrp/tcsetpgrp
  - string: strsignal
  - stdlib: bsearch, div/ldiv, mkstemp, strtod/strtof
  - stdio: scanf, fscanf, tmpfile, tmpnam
  - unistd: execle, getlogin, confstr, sbrk, link/symlink/readlink wrappers
  - New syslog.h stub (openlog/syslog/closelog)

Tier 4: Kernel syscalls:
  - umount2 (SYSCALL_UMOUNT2=138) with vfs_umount in fs.c
  - wait4 (SYSCALL_WAIT4=139) wrapping process_waitpid + rusage

Tier 5: Medium implementations:
  - glob.h/wordexp.h with directory walk + fnmatch pattern matching
  - pthread_mutex/cond/rwlock/key/once/barrier (futex-based)
  - /etc/passwd + /etc/group file parsing with static root fallback

Build: make clean && make OK, cppcheck clean, 101/101 smoke tests pass