Pure-reference catalog of payment lifecycle state machines across Stripe, Adyen, PayPal, and Braintree: canonical states (created / requires_action / processing / succeeded / requires_capture / canceled / failed), authorisation vs capture, asynchronous webhook states, refund / dispute / chargeback transitions, and the 3-D Secure (EMVCo 3DS 2.x) frictionless / challenge flow paths with per-gateway 3DS test cards (references/3ds-flows.md). Use when designing tests for payment flows, auditing state-handling code, or covering a 3DS round-trip; this is the state model, not a builder - to author suites on it use payment-flow-test-author (refunds, disputes, webhook replay).
