Summary

This PR implements a comprehensive check detection system for the chess engine, addressing issue #76.

New Methods Added to ChessBoard

• get_king_position(color): Returns the (row, col) tuple of the specified color king
• is_square_attacked(square, by_color): Returns True if any piece of the attacking color can reach the given square
• is_in_check(color): Returns True if the king of the specified color is currently in check
• would_be_in_check(move, color): Simulates a move and checks if it would leave the king in check

Updated Move Validation

• can_move_piece(): Now filters out any move that would leave the king in check
• castling_check(): Updated to enforce all castling rules (cannot castle out of, through, or into check)

Game State

• Added in_check boolean to the game state JSON

How to Test

1. Basic Check Detection

   • Load FEN: rnbqkbnr/pppp1ppp/8/4p3/6P1/5P2/PPPPP2P/RNBQKBNR b KQkq - 0 2
   • Move black queen to h4 (Qh4+) - white king should be in check
   • White should only be able to make moves that escape check

2. Pinned Piece

   • Load FEN: rnb1kbnr/pppp1ppp/8/4p3/7q/5NP1/PPPPP2P/RNBQKB1R w KQkq - 0 3
   • The g3 pawn is pinned - try moving it (should be blocked)

3. Castling Through Check

   • Load FEN: r3k2r/pppp1ppp/8/4p3/1b6/8/PPPPPPPP/R3K2R w KQkq - 0 1
   • White cannot castle queenside (d1 is attacked by bishop)

Closes #76