fix(io): add error handling for missing piece in PgnExporter replay

Replace unsafe fallback to Pawn with explicit exception on invariant
violation. This ensures data corruption in GameContext.moves is detected
immediately rather than producing silently incorrect PGN output.

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

modules/io/src/main/scala/de/nowchess/io/pgn/PgnExporter.scala

@@ -22,7 +22,15 @@ object PgnExporter extends GameContextExport:
     var ctx = GameContext.initial
     for move <- context.moves do
       val color = ctx.turn
-      val pieceType = ctx.board.pieceAt(move.from).map(_.pieceType).getOrElse(PieceType.Pawn)
+      val pieceType = ctx.board.pieceAt(move.from)
+        .map(_.pieceType)
+        .getOrElse {
+          val fromStr = move.from.toString
+          val boardStr = ctx.board.pieces.keys.map(_.toString).mkString(",")
+          throw new IllegalStateException(
+            s"Invariant violation: no piece at $fromStr during PGN export replay. Board squares: $boardStr"
+          )
+        }
       val isCapture = ctx.board.pieceAt(move.to).isDefined || move.moveType == MoveType.EnPassant
       val castleSide = move.moveType match
         case MoveType.CastleKingside  => Some("Kingside")