Adds SquareClient.DeleteCustomer for GDPR erasure (DELETE /v2/customers/{id}, NOT_FOUND as no-op), bounds doJSON response reads to 1 MiB with rune-safe 500-byte error snippets, adds validCardID guard to the disable-card URL, makes paymentFromSquare card fields consistent when the card ID is empty, redacts ccof/cnon tokens in all log paths, and fixes the idempotency-key-length comment (45 chars for payments/refunds/cards, 64 only for terminal checkouts).