feat: refactor move application logic to improve clarity and maintainability

modules/core/src/main/scala/de/nowchess/chess/notation/PgnParser.scala

@@ -42,26 +42,30 @@ object PgnParser:
         if isMoveNumberOrResult(token) then state
         else
           parseAlgebraicMove(token, board, history, color) match
-            case None => state  // unrecognised token — skip silently
+            case None       => state  // unrecognised token — skip silently
             case Some(move) =>
-              val newBoard = move.castleSide match
-                case Some(side) => board.withCastle(color, side)
-                case None =>
-                  val (boardAfterMove, _) = board.withMove(move.from, move.to)
-                  move.promotionPiece match
-                    case Some(pp) =>
-                      val pieceType = pp match
-                        case PromotionPiece.Queen  => PieceType.Queen
-                        case PromotionPiece.Rook   => PieceType.Rook
-                        case PromotionPiece.Bishop => PieceType.Bishop
-                        case PromotionPiece.Knight => PieceType.Knight
-                      boardAfterMove.updated(move.to, Piece(color, pieceType))
-                    case None => boardAfterMove
+              val newBoard   = applyMoveToBoard(board, move, color)
               val newHistory = history.addMove(move)
               (newBoard, newHistory, color.opposite, acc :+ move)
 
     moves
 
+  /** Apply a single HistoryMove to a Board, handling castling and promotion. */
+  private def applyMoveToBoard(board: Board, move: HistoryMove, color: Color): Board =
+    move.castleSide match
+      case Some(side) => board.withCastle(color, side)
+      case None =>
+        val (boardAfterMove, _) = board.withMove(move.from, move.to)
+        move.promotionPiece match
+          case Some(pp) =>
+            val pieceType = pp match
+              case PromotionPiece.Queen  => PieceType.Queen
+              case PromotionPiece.Rook   => PieceType.Rook
+              case PromotionPiece.Bishop => PieceType.Bishop
+              case PromotionPiece.Knight => PieceType.Knight
+            boardAfterMove.updated(move.to, Piece(color, pieceType))
+          case None => boardAfterMove
+
   /** True for move-number tokens ("1.", "12.") and PGN result tokens. */
   private def isMoveNumberOrResult(token: String): Boolean =
     token.matches("""\d+\.""") ||