Complete ECDSA Batch Verification Optimization

[kevinamccorvon]

Batch verification for ECDSA signatures is partially done but not optimized for P-384, P-521, and secp256k1. The performance gains here are massive - we're talking 3-10x faster signature verification.

What's missing:

• hpcrypt-signatures/ecdsa_p384.rs - batch verification exists but isn't optimized
• hpcrypt-signatures/ecdsa_p521.rs - precomputed tables not implemented
• hpcrypt-signatures/ecdsa_secp256k1.rs - needs optimized batch verification

The work:

1. Finish optimized batch verification for all three curves
2. Add precomputed tables where beneficial
3. Run benchmarks to measure improvements
4. Write comprehensive tests
5. Document the API and show performance numbers

This is a big win for blockchain nodes, certificate validation servers, and anything doing high-volume signature checks.