refactor: migrate devfs, procfs, tmpfs, overlayfs, persistfs to inode_operations

refactor: migrate devfs, procfs, tmpfs, overlayfs, persistfs to inode_operations

- devfs: devfs_dir_iops with lookup/readdir; devfs_dir_ops now empty
- procfs: procfs_root_iops, procfs_self_iops, procfs_pid_dir_iops
  with lookup/readdir; corresponding fops now empty
- tmpfs: tmpfs_dir_iops with lookup/readdir; tmpfs_dir_ops now empty;
  all dir creation sites (tmpfs_child_ensure_dir, tmpfs_create_root)
  wire i_ops
- overlayfs: overlay_dir_iops with lookup/readdir; finddir_impl and
  readdir_impl updated to check i_ops->lookup/readdir on child layers
  before falling back to f_ops (needed since child FSes now use i_ops)
- persistfs: persistfs_root_iops with lookup

All file-type nodes (read/write/poll/ioctl) remain in f_ops only —
correct separation of concerns.

20/20 smoke tests pass.