refactor: migrate pty and fat to inode_operations

refactor: migrate pty and fat to inode_operations

- pty: pty_pts_dir_iops with lookup/readdir; pty_pts_dir_fops now empty
- fat: fat_dir_iops with lookup/readdir/create/mkdir/unlink/rmdir/rename;
  fat_file_iops with truncate; fat_dir_fops and fat_file_fops keep only
  close and read/write/close respectively
- ext2 has no VFS integration yet, no migration needed

All node creation sites wire both f_ops and i_ops.

20/20 smoke tests pass.