Decision procedure for a merchant adding a new settlement rail to a live x402-metered endpoint. Covers the CDN stale-cha

Decision procedure for a merchant adding a new settlement rail to a live x402-metered endpoint. Covers the CDN stale-challenge problem (edge caches serving old 402 payment terms), pre-flight capability evidence for the new rail, staged rollout with old rail kept active, cross-region 402 challenge verification, and explicit rollback criteria. Output: PASS with rollout window or ROLLBACK with evidence. Built for operators who have been burned debugging stale CDN caches after announcing a new rail