Close the coverage-gap round: terminal CreateCheckout-failure marks the provisional row failed, GetCheckoutStatus reference_id mismatch 400, deadline wire shape, concurrent loyalty redemption 409, delete_guest_user + stale-guest saved-card scrubbing, ValidateAmount and isTokenLike direct units, bounded try-lock timeout, buildSplitRecords tip-overflow, and concurrent same-key dedup for gift card / booking / tip / checkout.
