The parser now captures the source 837's ST02 (transaction set control
number) on the Envelope model as 'transaction_set_control_number'.
add_record reads it off the envelope and stores it on the Batch row,
mirroring how 'envelope.control_number' (ISA13) was already handled.
Unlocks the SP37 join-key update so 999 set_control_number (AK201)
values resolve back to the right batch via Pass 1.
Nora