2026. Three Patterns for 99.9% Reliability in Distributed Payments: Velocity-Aware Idempotency, Cross-Protocol Failover, and Shadow-Compare Migration. International Journal of Engineering Research and Science & Technology. 22, 3 (Jul. 2026), 64–79. DOI:https://doi.org/10.62643/ijerst.2026.v22.n3.3800.