feat: symbolic links (symlink, readlink) and link stub

feat: symbolic links (symlink, readlink) and link stub

- include/fs.h: added FS_SYMLINK type and symlink_target[128] field to fs_node_t
- include/stat.h: added S_IFLNK define
- include/syscall.h: added SYSCALL_LINK(54), SYSCALL_SYMLINK(55), SYSCALL_READLINK(56)
- src/kernel/fs.c: vfs_lookup follows symlinks with depth limit (max 8)
- src/kernel/tmpfs.c: tmpfs_create_symlink creates FS_SYMLINK nodes
- src/kernel/syscall.c: symlink_impl, readlink_impl, link_impl (stub -ENOSYS)
  stat_from_node reports S_IFLNK for symlink nodes
- cppcheck clean, 19/19 smoke tests pass