Add CLAUDE.md documentation for AI assistants

Documents architectural patterns, conventions, and lessons learned for
working with the GeoIP2-java codebase. Includes guidance on records,
Jackson configuration, deprecation patterns, testing conventions, and
common pitfalls with solutions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: oschwald