Implement Check Detection System

[walshb421]

Summary

Implement check detection to prevent illegal moves that leave the king in check. This is a prerequisite for checkmate/stalemate detection and is essential for a rules-compliant chess game.

Currently, players can:

• Move their king into check
• Ignore being in check
• Move pinned pieces freely

Implementation Requirements

1. Add get_king_position(color) method to ChessBoard

• Returns the (row, col) tuple of the specified color's king
• Should iterate through the board to find the king piece

2. Add is_square_attacked(square, by_color) method to ChessBoard

• Returns True if any piece of by_color can attack the given square
• Must check all opponent pieces and their possible moves
• Used for both check detection and castling validation

3. Add is_in_check(color) method to ChessBoard

• Returns True if the king of the specified color is in check
• Utilizes get_king_position() and is_square_attacked()

4. Add would_be_in_check(move, color) method to ChessBoard

• Simulates the move on a copy of the board
• Returns True if the move would leave the king in check
• Must handle special cases: castling through check, en passant discovered check

5. Filter illegal moves in can_move_piece()

• After generating possible moves, filter out any that would leave the king in check
• Update validate_move() to use the new check detection

6. Update castling validation in castling_check()

• King cannot castle out of check
• King cannot castle through check (squares king passes over)
• King cannot castle into check
• Replace the existing TODO comment with actual implementation

Acceptance Criteria

• King cannot move to a square that is attacked
• Pieces pinned to the king cannot move (unless along the pin line)
• When in check, only moves that escape check are allowed
• Castling is blocked if king is in check, passes through check, or ends in check
• All existing move generation tests still pass

Technical Notes

• Consider creating a simulate_move() helper that returns a board copy with a move applied
• Be careful with performance - check detection runs for every move validation
• The generate_moves() methods in piece classes should remain unchanged; filtering happens at the board level