feat: improve PGN loading with error handling for promotions and add tests for en passant capture

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -221,17 +221,40 @@ class GameEngine(
       case Left(err) =>
         Left(err)
       case Right(game) =>
+        val initialBoardBeforeLoad = currentBoard
+        val initialHistoryBeforeLoad = currentHistory
+        val initialTurnBeforeLoad = currentTurn
+        
         currentBoard = Board.initial
         currentHistory = GameHistory.empty
         currentTurn = Color.White
         pendingPromotion = None
         invoker.clear()
-        game.moves.foreach { move =>
-          handleParsedMove(move.from, move.to, s"${move.from}${move.to}")
-          move.promotionPiece.foreach(completePromotion)
+
+        var error: Option[String] = None
+        import scala.util.control.Breaks._
+        breakable {
+          game.moves.foreach { move =>
+            handleParsedMove(move.from, move.to, s"${move.from}${move.to}")
+            move.promotionPiece.foreach(completePromotion)
+            
+            // If the move failed to execute properly, stop and report
+            // (validatePgn should have caught this, but we're being safe)
+            if pendingPromotion.isDefined && move.promotionPiece.isEmpty then
+              error = Some(s"Promotion required for move ${move.from}${move.to}")
+              break()
+          }
         }
-        notifyObservers(PgnLoadedEvent(currentBoard, currentHistory, currentTurn))
-        Right(())
+        
+        error match
+          case Some(err) =>
+            currentBoard = initialBoardBeforeLoad
+            currentHistory = initialHistoryBeforeLoad
+            currentTurn = initialTurnBeforeLoad
+            Left(err)
+          case None =>
+            notifyObservers(PgnLoadedEvent(currentBoard, currentHistory, currentTurn))
+            Right(())
   }
 
   /** Load an arbitrary board position, clearing all history and undo/redo state. */

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

@@ -73,7 +73,7 @@ object MoveValidator:
     val fi       = from.file.ordinal
     val ri       = from.rank.ordinal
     val dir      = if color == Color.White then 1 else -1
-    val startRank = if color == Color.White then 1 else 6  // R2 = ordinal 1, R7 = ordinal 6
+    val startRank = if color == Color.White then Rank.R2.ordinal else Rank.R7.ordinal
 
     val oneStep = squareAt(fi, ri + dir)
 

modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineLoadPgnTest.scala

@@ -1,7 +1,7 @@
 package de.nowchess.chess.engine
 
 import scala.collection.mutable
-import de.nowchess.api.board.{Board, Color}
+import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
 import de.nowchess.chess.logic.GameHistory
 import de.nowchess.chess.observer.*
 import org.scalatest.funsuite.AnyFunSuite
@@ -91,10 +91,22 @@ class GameEngineLoadPgnTest extends AnyFunSuite with Matchers:
     val pgn =
       """[Event "T"]
 
-1. a3 h6 2. a4 h5 3. a5 h4 4. a6 h3 5. a7 h2 6. a8=Q
+1. b4 h6 2. b5 h5 3. b6 h4 4. bxa7 h3 5. a8=Q
 """
     engine.loadPgn(pgn) shouldBe Right(())
-    engine.history.moves.length shouldBe 12
+    engine.history.moves.length shouldBe 9
+
+  test("loadPgn: en passant capture in PGN is replayed correctly"):
+    val engine = new GameEngine()
+    val pgn =
+      """[Event "T"]
+
+1. e4 e6 2. e5 d5 3. exd6
+"""
+    engine.loadPgn(pgn) shouldBe Right(())
+    engine.history.moves.length shouldBe 5
+    engine.board.pieceAt(Square(File.D, Rank.R6)) shouldBe Some(Piece(Color.White, PieceType.Pawn))
+    engine.board.pieceAt(Square(File.D, Rank.R5)) shouldBe None
 
   // ── undo/redo notation events ─────────────────────────────────────────────
 
@@ -150,9 +162,8 @@ class GameEngineLoadPgnTest extends AnyFunSuite with Matchers:
     val engine = new GameEngine()
     val cap    = new EventCapture()
     engine.subscribe(cap)
-    // e4, e5, then Black plays d5, White pawn on e4 captures on d5
+    // e4, then Black plays d5, White pawn on e4 captures on d5
     engine.processUserInput("e2e4")
-    engine.processUserInput("e7e5")
     engine.processUserInput("d7d5")
     engine.processUserInput("e4d5")  // white pawn captures black pawn
     engine.undo()

modules/core/src/test/scala/de/nowchess/chess/notation/PgnValidatorTest.scala

@@ -100,7 +100,7 @@ class PgnValidatorTest extends AnyFunSuite with Matchers:
     val pgn =
       """[Event "Test"]
 
-1. a4 h6 2. a5 h5 3. a6 h4 4. a7 h3 5. a8=Q h2
+1. b4 h6 2. b5 h5 3. b6 h4 4. bxa7 h3 5. a8=Q
 """
     PgnParser.validatePgn(pgn) match
       case Right(game) =>
@@ -108,6 +108,20 @@ class PgnValidatorTest extends AnyFunSuite with Matchers:
         game.moves.last.promotionPiece shouldBe Some(PromotionPiece.Queen)
       case Left(err) => fail(s"Expected Right but got Left($err)")
 
+  test("validatePgn: en passant capture is parsed correctly"):
+    val pgn =
+      """[Event "Test"]
+
+1. e4 e6 2. e5 d5 3. exd6
+"""
+    PgnParser.validatePgn(pgn) match
+      case Right(game) =>
+        game.moves.length shouldBe 5
+        val epMove = game.moves.last
+        epMove.isCapture shouldBe true
+        epMove.pieceType shouldBe PieceType.Pawn
+      case Left(err) => fail(s"Expected Right but got Left($err)")
+
   test("validatePgn: disambiguation with two rooks is accepted"):
     val pieces: Map[Square, Piece] = Map(
       Square(File.A, Rank.R1) -> Piece(Color.White, PieceType.Rook),