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/vectors.json

@@ -478,5 +478,51 @@
         }
       }
     }
+  ],
+  "bip85": [
+    {
+      "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,
+      "bip85_path": "m/83696968'/39'/0'/12'/0'",
+      "expected_entropy64_hex": "ac98dac5d4f4ebad6056682ac95eb9ad9ba94fb68e96848264dad0b4357d002e41b3dd7a4c6f4ebc234be6938495840a73f59e9ba0e8e5c5208c94e6df2d7709",
+      "expected_entropy_truncated_hex": "ac98dac5d4f4ebad6056682ac95eb9ad",
+      "expected_child_mnemonic": "prosper short ramp prepare exchange stove life snack client enough purpose fold"
+    },
+    {
+      "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,
+      "bip85_path": "m/83696968'/39'/0'/18'/0'",
+      "expected_entropy64_hex": "fc039f51d67ed7dfd01552f27de28887cf3e58655153e44b023d37578321f7083241970730e522d3f20b38a5296c5e51e57e0429546629704a09c6d1e2d10829",
+      "expected_entropy_truncated_hex": "fc039f51d67ed7dfd01552f27de28887cf3e58655153e44b",
+      "expected_child_mnemonic": "winter brother stamp provide uniform useful doctor prevent venue upper peasant auto view club next clerk tone fox"
+    },
+    {
+      "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,
+      "bip85_path": "m/83696968'/39'/0'/24'/0'",
+      "expected_entropy64_hex": "d5a9cb46670566c4246b6e7af22e1dfc3668744ed831afea7ce2beea44e34e23e348e86091f24394f4be6253a7d5d24b91b1c4e0863b296e9e541e8018288897",
+      "expected_entropy_truncated_hex": "d5a9cb46670566c4246b6e7af22e1dfc3668744ed831afea7ce2beea44e34e23",
+      "expected_child_mnemonic": "stick exact spice sock filter ginger museum horse kit multiply manual wear grief demand derive alert quiz fault december lava picture immune decade jaguar"
+    },
+    {
+      "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,
+      "bip85_path": "m/83696968'/39'/0'/12'/0'",
+      "expected_entropy64_hex": "2b1d7c4f311137fa95f6302e64cdb88584d52b51b57d0430ee68e148b82baa3f8c40316397eb404f4573bc0c8e5c4bc14e4aa5f0f472a9d3587f494f1f7b3684",
+      "expected_entropy_truncated_hex": "2b1d7c4f311137fa95f6302e64cdb885",
+      "expected_child_mnemonic": "climb typical because giraffe beach wool fit ship common chapter hotel arm"
+    }
   ]
 }