Reserve credit against a payment for refund

Reserves unused purchased execution credit against one original payment, with the billing owner's explicit confirmation. Purchased credit is not refundable except where a charge is refunded because the service did not work as described, so this records no promise of a refund: it never calls Stripe, cashes out credit or grants an MCP cash-out path. A Stripe refund of that payment settles the reservation when it carries the same credit and supersedes it otherwise; the Idempotency-Key preserves one reservation across retries. Trial credit has no cash value, and a refund does not restore automatic purchasing authority used in the month.