Add system property like __id back to entity CSV attachment?

I was thinking about this [comment](https://forum.getodk.org/t/when-editing-a-submission-that-created-an-entity-my-uniqueness-check-says-that-entity-was-already-created-so-i-cant-edit-submitted-data/44559/10?u=ktuite) on the forum about how someone fixed a problem with their form by replacing `__id` with `name`.

Because we build up the entity CSV differently for downloading it directly vs. attaching it to a form (though at one point they were the same), there isn't a way to directly look at the entity CSV that gets attached to the form.

Maybe we should document this better OR change the attachment CSV a little bit.