Add BIP32 path derivation foundation

[mtg0001]

Summary

• Add general BIP32 private child derivation for both hardened and non-hardened child numbers.
• Add BIP32DerivePath() so wallet code can derive multi-level paths instead of only the current single hardened child.
• Keep the existing BIP32DeriveHardenedChild(parent, index) wrapper intact for the current wallet path.
• Extend test_bip32 with official BIP32 test vector 1 paths, including mixed hardened/non-hardened derivation.

Why

BIP44 cannot be wired safely until the BIP32 layer can derive paths such as m/44'/coin_type'/0'/0/i and m/44'/coin_type'/0'/1/i. The current wallet only needs m/index', so this PR deliberately does not change wallet behavior, stored metadata, restore scanning, or address generation yet.

This is the foundation PR: it makes the cryptographic path machinery testable before the wallet starts using a BIP44 layout.

Testing

• git diff --check
• make -f makefile.mingw test_bip32.exe && ./test_bip32.exe from MSYS2 UCRT64 in src/
• make -f makefile.mingw tests from MSYS2 UCRT64 in src/

[Bitflash-sh]

Landed in #124, which contains this branch — the four were stacked, so squashing the top brought all of them into main as 876d7a7. Closing this one as merged rather than leaving it open against changes that are already in.

The real-coin verification is written up on #124.