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/generate_bip85_vectors.py

@@ -0,0 +1,91 @@
+#!/usr/bin/env python3
+"""
+Generate BIP85 test vectors for vectors.json
+
+Run this from the repo root:
+    python3 tests/generate_bip85_vectors.py
+"""
+
+import sys
+from pathlib import Path
+
+# Add src to path
+sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
+
+import pyhdwallet
+from bip_utils import Bip39SeedGenerator
+import json
+
+# Test cases to generate
+test_cases = [
+    {
+        "description": "BIP85 test case 1: 12-word child, no passphrase",
+        "master_mnemonic": "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+        "master_passphrase": "",
+        "child_words": 12,
+        "index": 0,
+    },
+    {
+        "description": "BIP85 test case 2: 18-word child, no passphrase",
+        "master_mnemonic": "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+        "master_passphrase": "",
+        "child_words": 18,
+        "index": 0,
+    },
+    {
+        "description": "BIP85 test case 3: 24-word child, no passphrase",
+        "master_mnemonic": "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+        "master_passphrase": "",
+        "child_words": 24,
+        "index": 0,
+    },
+    {
+        "description": "BIP85 test case 4: 12-word child, WITH passphrase",
+        "master_mnemonic": "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+        "master_passphrase": "TREZOR",
+        "child_words": 12,
+        "index": 0,
+    },
+]
+
+print("Generating BIP85 test vectors...\n")
+print("=" * 80)
+
+vectors = []
+
+for case in test_cases:
+    print(f"\n{case['description']}")
+    print("-" * 80)
+    
+    # Generate master seed
+    master_seed = Bip39SeedGenerator(case["master_mnemonic"]).Generate(case["master_passphrase"])
+    
+    # Derive child using BIP85
+    path, child_mnemonic, entropy64, truncated_entropy = pyhdwallet.bip85_derive_child_mnemonic(
+        master_seed,
+        case["child_words"],
+        case["index"]
+    )
+    
+    vector = {
+        "description": case["description"],
+        "master_mnemonic": case["master_mnemonic"],
+        "master_passphrase": case["master_passphrase"],
+        "child_words": case["child_words"],
+        "index": case["index"],
+        "bip85_path": path,
+        "expected_entropy64_hex": entropy64.hex(),
+        "expected_entropy_truncated_hex": truncated_entropy.hex(),
+        "expected_child_mnemonic": child_mnemonic,
+    }
+    
+    vectors.append(vector)
+    
+    print(f"Path: {path}")
+    print(f"Entropy64: {entropy64.hex()}")
+    print(f"Truncated: {truncated_entropy.hex()}")
+    print(f"Child mnemonic: {child_mnemonic}")
+
+print("\n" + "=" * 80)
+print("\nJSON output for vectors.json:\n")
+print(json.dumps(vectors, indent=2))