Legal castling

[Hardcode3]

🎯 Goal

Generate castling moves correctly.

🧠 Key Idea

Castling is a special rule, not a normal move.

🛠️ Tasks

• Verify castling rights
• Verify empty path
• Verify non-attacked squares
• Emit castling move

✅ Acceptance Criteria

• No illegal castling
• Fully perft-compatible