Account-credit refunds now require the original payment and venue credit balance
to use the same currency. A payment in another currency must be refunded through
its original payment method. If a refund is already pending, keep its existing
reference for reconciliation. Completed refund retries reuse the recorded result.

POS refund amounts also preserve the original payment's currency precision,
including currencies with no decimal places or three decimal places. The
remaining refundable amount is no longer rounded to two decimals in the register.