Complete BIP-32/BIP-39/BIP-44 HD wallet implementation #18

Description

Implement full hierarchical deterministic wallet support.

Location

• File: src/signer/wallet.zig
• Lines: 266, 283, 290, 307

Current Status

Framework exists but derivation is stubbed.

Tasks

• Implement BIP-32 child key derivation (CKD)
• Implement hardened derivation
• Implement non-hardened derivation
• Implement BIP-44 paths (m/44'/60'/0'/0/x)
• Add complete BIP-39 word list (2048 words)
• Implement mnemonic generation from entropy
• Implement BIP-39 checksum validation
• Add comprehensive tests

Priority

High - Essential for full HD wallet support

References

• BIP-32: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
• BIP-39: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
• BIP-44: https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki