feat: verify optimized Keccacheck settlement proofs - #37

Summary

• replaces the embedded Groth16 verifying key with the key exported by MiragePrivacy/circuits#2
• vendors a real proof generated by the optimized 4,034,927-constraint Keccacheck receipt circuit
• preserves the existing collect ABI and two public statement limbs

Validation

• EscrowZKTest: 9 passed, 0 failed
• real optimized proof settles successfully
• tampered proof and public signals are rejected
• measured ZK collect gas: 258,354 in the fixture test

Stack

• base ZK escrow: feat: ERC-20 ZK verifier and statement hash #36
• optimized circuit: MiragePrivacy/circuits#2
• matching Nomad SGX integration PR will be linked here

Draft until the circuit transcript is audited and the Nomad Fortanix build plus SGX hardware E2E completes.