Split wallet.cpp: extract key management into wallet_keys.cpp
Separate key management from transaction logic in wallet.cpp (5,059 lines):
- wallet.cpp: 5,059 → 4,143 lines (transaction tracking, coin selection, balances)
- wallet_keys.cpp: 975 lines (key generation, encryption, HD seed, keypool)
Extracted functions include key generation (GenerateNewKey, GenerateNewSaplingZKey),
cryptographic key storage (AddKeyPubKey, AddCryptedKey, AddCryptedSaplingSpendingKey),
wallet encryption (Unlock, ChangeWalletPassphrase, EncryptWallet), HD seed management,
keypool operations, CReserveKey methods, and shielded key visitor operators.
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