Submit a single signed transaction via Astralane's MEV-protected gateway. The server fans out to multiple geo-distributed Astralane endpoints (NY, FR, SG, JP, AMS, LIM, LA) and returns the first success.

MEV Protection Flow:

1. Build tx via /positions/open or /positions/close with astralaneTipLamports param
2. Sign the returned transaction client-side
3. Submit here with mevProtect: true

The transaction must be base64-encoded.