- encoder/mh.rs: Full T.4 Group 3 MH encoding implementation
- Uses ITU-T T.4 standard Huffman tables for white/black runs
- Adds EOL codes and RTC (6 EOLs) at document end
- Class2Send now encodes pixels to MH before transmission
- CLI convert command shows MH encoded size
Test result: 467KB raw pixels → 14.6KB MH encoded (~32:1 compression)