feat: implement half-half attack (single signature)

[oritwoen]

Description

Implement the half-half attack where k = h_msb || d_msb (nonce constructed from hash and private key MSBs).

Background

In 2023, researchers discovered 88,230 vulnerable Bitcoin signatures where the nonce was constructed by concatenating MSBs of the message hash and private key. This allows key recovery from a single signature.

Impact: 222 BTC were at risk.

Requirements

• Detect half-half nonce pattern
• Single-signature key recovery
• Support various bit-split ratios (128|128, etc.)

Technical Notes

• Most dangerous attack - only needs 1 signature
• Detection may require brute-force verification
• Consider GPU acceleration for detection phase