Legal pawn moves

🎯 Goal

Handle the most complex piece correctly.

🧠 Key Idea

Pawns obey the same masks + special rules.

🛠️ Tasks

- Single and double pushes
- Captures
- Promotions
- En passant:
- discovered check handling
- pinned pawn handling

✅ Acceptance Criteria

- EP edge cases covered
- Promotions are legal only
