docs: add handover and update lessons learned for issue #394

Add comprehensive handover document covering:
- Complete implementation summary
- Technical architecture changes
- Testing coverage and verification
- Next steps and recommendations

Update lessons-learned.md with 6 new patterns:
1. Separate storage from display in normalized structures
2. URL protection via masking-replacement-restoration
3. Regex escape sequences in character classes
4. Disciplined development phases prevent scope creep
5. Struct literal updates after adding optional fields
6. LazyLock for thread-safe static regex

Related: #394, #395

Author: AlexMikhalev