feat: add cross-platform MDB generation using Java Jackcess
- Created Java MDB generator for generating Microsoft Access database files
- Uses Jackcess 4.0.5 library for cross-platform MDB creation (works on Linux/macOS/Windows)
- Reads UNLOCODE CSV files (3-part split format) and generates Access 2003 format MDB
- Added Maven build configuration with executable JAR output
- Updated GitHub Actions workflow to:
- Install Java 11 and build MDB generator
- Generate MDB files after CSV generation
- Include MDB in release tarballs
- Added comprehensive README documentation for the MDB generator
- Resolves cross-platform MDB generation issue (Python mdbtools doesn't work on Linux)