feat: permissions support (uid/gid/mode, chmod, chown, getuid, getgid)

feat: permissions support (uid/gid/mode, chmod, chown, getuid, getgid)

- include/fs.h: added uid, gid, mode fields to fs_node_t
- include/process.h: added uid, gid fields to struct process
- include/stat.h: added st_uid, st_gid to struct stat, permission bit defines
- include/syscall.h: added SYSCALL_CHMOD(50), SYSCALL_CHOWN(51), SYSCALL_GETUID(52), SYSCALL_GETGID(53)
- src/kernel/syscall.c: chmod_impl, chown_impl, getuid/getgid dispatch; stat_from_node now populates uid/gid/mode
- user/init.c: updated struct stat to match kernel layout
- cppcheck clean, 19/19 smoke tests pass