Check evasion mask (check_mask)

[Hardcode3]

🎯 Goal

Restrict legal moves while in check.

🧠 Key Idea

Situation	check_mask
No check	FULL
Non-sliding check	Checker square
Sliding check	Ray king <-> checker
Double check	EMPTY

🛠️ Tasks

• Implement ray_between(a, b)
• Identify checker type
• Build check_mask

✅ Acceptance Criteria

• Only moves that resolve check are generated
• No illegal evasions allowed