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

pytest.ini

@@ -0,0 +1,7 @@
+[pytest]
+filterwarnings =
+    ignore::DeprecationWarning:pgpy.constants
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*