Validate payment OK before allowing process/next

We need to check that the payment is completed before we allow moving to the next process step, as long as it's not a zero sum order.

Start by tracing the process/next transition and how payment completion is represented. Confirm the behavior for non-zero-sum orders and verify that zero-sum orders can still advance; done means an unpaid non-zero order cannot move to the next process step.

Written by the indexing model from the issue text.