Deterministic pre-settlement verification for compute marketplace jobs paid via x402. Eight checks: signed execution rec

Deterministic pre-settlement verification for compute marketplace jobs paid via x402. Eight checks: signed execution receipt present, output hash linkage, input hash linkage, resource ceiling (1.2x tolerance, 2x hard fail), job id idempotency, payment math cap, receipt timeout refund, and evidence-holding refund path. Pay only after all checks pass.