[P3][L] Add first-class Solana/SVM payment path in demo + zkx402

[markballew]

Summary

Demonstrate and support Solana payments (x402 SVM exact scheme) alongside EVM in zkx402.

Acceptance criteria

• At least one endpoint supports multi-network accepts[] (EVM + Solana)
• Subject identity model does not assume EVM-only
• Demo shows a working SVM payment path (or a documented stub if missing prerequisites)