Ref T4807. In the next diff, which actually implements "natural" ordering, I need to be able to sort column positions.
Currently, the objects don't have enough information to do this on their own, since the complete ordering is <sequence, objectID> and I think it's desirable to be flexible about sequences being unique.
We could load the attached objects to figure out the objectID part, but this is a huge amount of data which is expensive to query.
Instead, just denormalize object IDs into the column position row. These always exist, are stable, and are easy to denormalize. We can then order all positions purely with the position objects.
(I didn't adjust the key on the table since we don't actually need it to include this column and I don't anticipate changing the query, at least for now.)