Check detection (checkers)

🎯 Goal

Detect king status:

- not in check
- single check
- double check

🧠 Key Idea

```cpp
checkers = attackers_to(king_square, enemy)

bool in_check        = !checkers.empty();
bool double_check    = popcount(checkers) >= 2;
```

🛠️ Tasks

- Compute checkers
- Expose:
  - in_check
  - double_check

✅ Acceptance Criteria

- Works for all piece types
- Handles discovered and double checks