feat: add SolPay facilitator

[benniethedev]

Add Facilitator

Summary

• Facilitator Name: SolPay
• URL: https://x402.solpay.cash
• Website: https://www.solpay.cash
• Twitter: @SolPayCash
• Short description: Solana-native x402 facilitator that sponsors SOL gas fees, allowing users to pay only
  in USDC for storefront purchases and API access.

Required changes

• Added config file at packages/facilitators/src/facilitators
• Exported in packages/facilitators/src/facilitators/index.ts
• Appended to packages/facilitators/src/lists/all.ts
• Chose a new unique color not used by other facilitators (#059669)
• Updated README.md with a one-line entry under Facilitators
• dateOfFirstTransaction is correct and matches on-chain history (2026-01-16)
• Facilitator has at least 10 USDC transfers
• Added image to apps/scan/public
• Make sure we support that chain (Solana ✓)
• Make sure all EVM addresses are lowercase (N/A - Solana only)

Notes

• Transactions here https://solscan.io/account/86Ts3pgt61316eCC8RR1bHoCgtLdt6BD3imrWXALWKtp
• Image (solpay.png) to be provided for apps/scan/public
• Facilitator is newly launched on mainnet (2026-01-16), working toward 10 USDC transfer threshold

[vercel]

@benniethedev is attempting to deploy a commit to the Merit Systems Team on Vercel.

A member of the Team first needs to authorize it.

[benniethedev]

Hello @shafu0x , is there anything else needed from my end for this PR? Thanks!