feat(bip85): add gen-child command (v1.0.6)

Implements BIP85 child mnemonic derivation with full interoperability.

Features:
- Derives child BIP39 mnemonics (12/15/18/21/24 words) from master mnemonic
- BIP85 path: m/83696968'/39'/0'/{words}'/{index}'
- Supports optional master BIP39 passphrase
- Reuses existing input modes (--interactive, --mnemonic-stdin)
- Follows existing UX patterns (--off-screen, --file, PGP encryption)
- Offline-first with NetworkGuard protection

Testing:
- Adds deterministic regression tests for BIP85 spec compliance
- Verified against official BIP85 test vectors
- CLI smoke tests for end-to-end validation

Interoperability:
- Produces mnemonics compatible with Coldcard, Ian Coleman tool, etc.
- Test vector verified: 'girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose'

Version bumped to v1.0.6

tests/test_vectors.py

@@ -89,6 +89,91 @@ def test_address_derivation_integrity(vectors):
             )
             assert result["addresses"] == expected_addresses, f"Mismatch for profile {profile}"
 
+def test_bip85_derivation(vectors):
+    """
+    Verifies BIP85 child mnemonic derivation matches expected test vectors.
+    
+    Tests:
+    - Path construction
+    - Entropy64 derivation (HMAC-SHA512)
+    - Entropy truncation
+    - Child mnemonic generation
+    """
+    from bip_utils import Bip39SeedGenerator
+    
+    for case in vectors["bip85"]:
+        master_mnemonic = case["master_mnemonic"]
+        master_passphrase = case["master_passphrase"]
+        child_words = case["child_words"]
+        index = case["index"]
+        
+        # Generate master seed
+        master_seed = Bip39SeedGenerator(master_mnemonic).Generate(master_passphrase)
+        
+        # Derive child using BIP85
+        path, child_mnemonic, entropy64, truncated_entropy = pyhdwallet.bip85_derive_child_mnemonic(
+            master_seed,
+            child_words,
+            index
+        )
+        
+        # Assert path
+        assert path == case["bip85_path"], f"Path mismatch: {path} != {case['bip85_path']}"
+        
+        # Assert entropy64 (full 64-byte HMAC output)
+        assert entropy64.hex() == case["expected_entropy64_hex"], \
+            f"Entropy64 mismatch for {case['description']}"
+        
+        # Assert truncated entropy
+        assert truncated_entropy.hex() == case["expected_entropy_truncated_hex"], \
+            f"Truncated entropy mismatch for {case['description']}"
+        
+        # Assert child mnemonic
+        assert child_mnemonic == case["expected_child_mnemonic"], \
+            f"Child mnemonic mismatch for {case['description']}"
+
+
+def test_cli_gen_child_smoke(tmp_path, vectors):
+    """
+    CLI smoke test for gen-child command.
+    
+    Verifies:
+    - Command runs without error
+    - ZIP file is created
+    - Off-screen mode works
+    - File output works
+    """
+    vector = vectors["bip85"][0]  # Use first BIP85 test case
+    master_mnemonic = vector["master_mnemonic"]
+    
+    cmd = [
+        sys.executable, "src/pyhdwallet.py", "gen-child",
+        "--mnemonic-stdin",
+        "--index", "0",
+        "--words", "12",
+        "--off-screen",
+        "--file",
+        "--zip-password-mode", "auto",
+        "--wallet-location", str(tmp_path)
+    ]
+    
+    result = subprocess.run(
+        cmd,
+        input=master_mnemonic.encode("utf-8"),
+        capture_output=True
+    )
+    
+    assert result.returncode == 0, f"CLI gen-child failed: {result.stderr.decode()}"
+    
+    # Check ZIP file created
+    zips = list(tmp_path.glob("*.zip"))
+    assert len(zips) == 1, f"Expected exactly one zip file, found {len(zips)}"
+    
+    # Check filename pattern
+    zip_name = zips[0].name
+    assert zip_name.startswith("bip85_child_"), f"Unexpected zip name: {zip_name}"
+
+
 def test_cli_recover_smoke(tmp_path, vectors):
     """
     Runs the CLI in a subprocess to verify end-to-end wiring