feat: implement GameRules with isInCheck, legalMoves, gameStatus

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala

@@ -8,10 +8,35 @@ enum PositionStatus:
 object GameRules:
 
   /** True if `color`'s king is under attack on this board. */
-  def isInCheck(board: Board, color: Color): Boolean = false
+  def isInCheck(board: Board, color: Color): Boolean =
+    board.pieces
+      .collectFirst { case (sq, p) if p.color == color && p.pieceType == PieceType.King => sq }
+      .exists { kingSq =>
+        board.pieces.exists { case (sq, piece) =>
+          piece.color != color &&
+          MoveValidator.legalTargets(board, sq).contains(kingSq)
+        }
+      }
 
   /** All (from, to) moves for `color` that do not leave their own king in check. */
-  def legalMoves(board: Board, color: Color): Set[(Square, Square)] = Set.empty
+  def legalMoves(board: Board, color: Color): Set[(Square, Square)] =
+    board.pieces
+      .collect { case (from, piece) if piece.color == color => from }
+      .flatMap { from =>
+        MoveValidator.legalTargets(board, from)
+          .filter { to =>
+            val (newBoard, _) = board.withMove(from, to)
+            !isInCheck(newBoard, color)
+          }
+          .map(to => from -> to)
+      }
+      .toSet
 
   /** Position status for the side whose turn it is (`color`). */
-  def gameStatus(board: Board, color: Color): PositionStatus = PositionStatus.Normal
+  def gameStatus(board: Board, color: Color): PositionStatus =
+    val moves   = legalMoves(board, color)
+    val inCheck = isInCheck(board, color)
+    if moves.isEmpty && inCheck then PositionStatus.Mated
+    else if moves.isEmpty       then PositionStatus.Drawn
+    else if inCheck             then PositionStatus.InCheck
+    else                             PositionStatus.Normal