Add externally verifiable test vectors for Electrum v1/v2 mnemonic derivation

[Copilot]

Adds comprehensive test vectors with independent verification for Electrum v1 and v2 mnemonic-to-key derivation. Previous implementation lacked external validation.

Test Vectors Added

bip39_tests.c:

• Electrum v2 (PBKDF2-HMAC-SHA512): versions 0x01 and 0x100, with/without passphrases
• Electrum v1 (SHA256 stretching): 12-word mnemonics, fallback paths
• All vectors include Python verification commands using standard library

key_tests.c:

• Electrum v1 custom derivation test using electrum_v1_derive_privkey32()
• Tests multiple keypaths: 0/0 (receiving), 0/1 (change), 5/0
• Previously incorrectly tested BIP32 HD derivation instead of Electrum v1's custom algorithm

Independent Verification

Python script (test/verify_electrum_v1_vectors.py):

from ecdsa import SECP256k1, SigningKey
from hashlib import sha256

# Electrum v1: privkey = (master_secret + SHA256(n:for_change:mpk)) mod order
sk = SigningKey.from_string(master_secret, curve=SECP256k1)
mpk = sk.get_verifying_key().to_string().hex()
prefix_str = f"{n}:{for_change}:{mpk}"
offset = sha256(prefix_str.encode()).digest()

Uses ecdsa library (official Electrum dependency) to independently verify all test vectors.

Key Fixes

• Electrum v1 test: Changed from dogecoin_hdnode_from_seed() (BIP32) to electrum_v1_derive_privkey32() (custom derivation)
• Keypath display: Fixed CLI to show electrum_v1:n/for_change format correctly (was reversed)
• Cross-compilation: Replaced strtok with strtok_r in bip39.c, spv.c, utils.c (fixes aarch64 linker errors)
• Test registration: Added test_electrum_v1_mnemonic_to_master_key() to unittester.c

Documentation

• doc/electrum_v1_keypath.md: Keypath format reference (electrum_v1:n/for_change)
• doc/electrum_v1_vs_bip32.md: Clarifies Electrum v1 uses custom derivation, not BIP32/BIP84 paths
• Enhanced function comments with verification methods and algorithm references

Verification Methods

All test vectors can be independently verified:

• Electrum v2: python3 -c "import hashlib; print(hashlib.pbkdf2_hmac('sha512', b'mnemonic', b'electrum', 2048).hex())"
• Electrum v1: python3 test/verify_electrum_v1_vectors.py
• CLI: ./such -c mnemonic_to_key -n "alpha bravo" -l matches test expectations

Test vectors reference NIST standards (PBKDF2: RFC 2898, SHA-256: FIPS 180-4) and official Electrum implementation.

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