feat: add Electrum-compatible wallet encryption

- Add src/crypto.py with AES-256-CBC pw_encode/pw_decode
  using double SHA256 key derivation (Electrum standard)
- Add encrypt_wallet() and decrypt_wallet() to src/hd_wallet.py
- Prompt for encryption password when saving HD wallet to file
- Add 13 encryption/decryption tests to tests/test_hd_wallet.py
- Add pycryptodome to requirements.txt

tests/test_hd_wallet.py

@@ -2,7 +2,8 @@ import sys
 import os
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 
-from src.hd_wallet import generate_hd_wallet
+from src.hd_wallet import generate_hd_wallet, encrypt_wallet, decrypt_wallet
+from src.crypto import pw_decode
 
 SECRETSCAN_URL = "https://secretscan.org/Bitcoin?address={}"
 
@@ -222,6 +223,103 @@ def test_hd_generates_mnemonic_when_none():
     assert len(result['keystore']['seed'].split()) == 12
 
 
+# --- Encryption ---
+
+TEST_PASSWORD = "correct_horse_battery_staple"
+
+def test_hd_no_encryption_by_default():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    assert result['use_encryption'] is False
+
+
+def test_hd_encryption_sets_flag():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    assert encrypted['use_encryption'] is True
+
+
+def test_hd_encryption_encrypts_seed():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    assert encrypted['keystore']['seed'] != TEST_MNEMONIC
+
+
+def test_hd_encryption_encrypts_xprv():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    original_xprv = result['keystore']['xprv']
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    assert encrypted['keystore']['xprv'] != original_xprv
+
+
+def test_hd_encryption_leaves_xpub_plaintext():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    assert encrypted['keystore']['xpub'] == result['keystore']['xpub']
+
+
+def test_hd_encryption_leaves_addresses_plaintext():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    assert encrypted['addresses']['receiving'][0]['address'] == result['addresses']['receiving'][0]['address']
+    assert encrypted['addresses']['receiving'][0]['public_key'] == result['addresses']['receiving'][0]['public_key']
+
+
+def test_hd_encryption_encrypts_private_keys():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=2)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    for i in range(2):
+        orig = result['addresses']['receiving'][i]
+        enc = encrypted['addresses']['receiving'][i]
+        assert enc['private_key_hex'] != orig['private_key_hex']
+        assert enc['private_key_wif'] != orig['private_key_wif']
+
+
+def test_hd_encryption_round_trip_seed():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    decrypted = decrypt_wallet(encrypted, TEST_PASSWORD)
+    assert decrypted['keystore']['seed'] == TEST_MNEMONIC
+
+
+def test_hd_encryption_round_trip_xprv():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    decrypted = decrypt_wallet(encrypted, TEST_PASSWORD)
+    assert decrypted['keystore']['xprv'] == result['keystore']['xprv']
+
+
+def test_hd_encryption_round_trip_private_keys():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=3)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    decrypted = decrypt_wallet(encrypted, TEST_PASSWORD)
+    for i in range(3):
+        assert decrypted['addresses']['receiving'][i]['private_key_hex'] == result['addresses']['receiving'][i]['private_key_hex']
+        assert decrypted['addresses']['receiving'][i]['private_key_wif'] == result['addresses']['receiving'][i]['private_key_wif']
+
+
+def test_hd_decrypt_wrong_password():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    encrypted = encrypt_wallet(result, TEST_PASSWORD)
+    try:
+        decrypt_wallet(encrypted, "wrong_password")
+        assert False, "Should have raised ValueError"
+    except ValueError:
+        pass
+
+
+def test_hd_encrypt_does_not_mutate_original():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    original_seed = result['keystore']['seed']
+    encrypt_wallet(result, TEST_PASSWORD)
+    assert result['keystore']['seed'] == original_seed
+
+
+def test_hd_decrypt_unencrypted_is_noop():
+    result = generate_hd_wallet('mainnet', 'bip84', TEST_MNEMONIC, num_addresses=1)
+    decrypted = decrypt_wallet(result, TEST_PASSWORD)
+    assert decrypted['keystore']['seed'] == result['keystore']['seed']
+
+
 # --- SecretScan ---
 
 def test_hd_print_secretscan(capsys):