The agentic payments landscape: x402 vs AP2, MPP, L402, and Solana Pay

x402 isn't the only way machines pay machines. A clear-eyed map of the agentic-payments stack in 2026 — x402, Google's AP2, Stripe/Tempo's MPP, Lightning's L402, Solana Pay, and card rails — what settles where, and where Solana fits.