I think the current model just adds the top-level site as an additional key.

However, this is inconsistent with plans for state elsewhere: privacycg/storage-partitioning#25. I think it would be good if there was alignment there. Otherwise we end up with a world where what is considered "partitioned" for some state is not considered "partitioned" for other state, which I suspect will lead to a lot of confusion and subtle application bugs.