- Use piece_sq * 6 + piece_type encoding - 32 active features for 32 pieces on board - Simplified from FullThreats (60,720) to HalfKAv2_hm only - All tests passing (11 tests)
- Implement piece-square feature extraction - 32 active features for 32 pieces on board - Tests for feature extraction (7 tests) - Fix: piece_sq * 6 + piece_type mapping