Mastercard Agent Pay for Machines: card rails for agents, permissions on Solana

Mastercard's AP4M (June 2026) lets AI agents transact at machine speed down to fractions of a cent. The design choice that matters: it stores agent permissions on public chains — Solana, Polygon, Base — while settling on Mastercard's own rails across fiat and stablecoins. What that split means next to the open x402 stack.