Legal sliding moves (bishop, rook, queen)

🎯 Goal

Generate sliding piece moves legal by construction.

🧠 Key Idea

```text
attacks
∩ ~own
∩ check_mask
∩ (pin_ray OR FULL)
```

🛠️ Tasks

- Implement:
  - generate_legal_bishop_moves
  - generate_legal_rook_moves
  - generate_legal_queen_moves
- No post-generation filtering

✅ Acceptance Criteria

- Pins fully respected
- No illegal interpositions