feat(io): implement PgnExporter.exportGameContext with move replay

PgnExporter now extends GameContextExport and implements exportGameContext,
which replays moves from GameContext.initial to reconstruct HistoryMove records
with proper castling/promotion info before generating PGN.

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

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

@@ -1,10 +1,41 @@
 package de.nowchess.io.pgn
 
 import de.nowchess.api.board.*
-import de.nowchess.api.move.PromotionPiece
-import de.nowchess.api.game.{GameHistory, HistoryMove}
+import de.nowchess.api.move.{PromotionPiece, MoveType}
+import de.nowchess.api.game.{GameHistory, HistoryMove, GameContext}
+import de.nowchess.io.GameContextExport
+import de.nowchess.rules.sets.DefaultRules
 
-object PgnExporter:
+object PgnExporter extends GameContextExport:
+
+  /** Export a GameContext to PGN format by replaying all moves and reconstructing HistoryMove records. */
+  def exportGameContext(context: GameContext): String =
+    val headers = Map(
+      "Event" -> "?",
+      "White" -> "?",
+      "Black" -> "?",
+      "Result" -> "*"
+    )
+
+    // Replay all moves to reconstruct HistoryMove records with full info
+    val historyMoves = scala.collection.mutable.ListBuffer[HistoryMove]()
+    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 isCapture = ctx.board.pieceAt(move.to).isDefined || move.moveType == MoveType.EnPassant
+      val castleSide = move.moveType match
+        case MoveType.CastleKingside  => Some("Kingside")
+        case MoveType.CastleQueenside => Some("Queenside")
+        case _                        => None
+      val promotionPiece = move.moveType match
+        case MoveType.Promotion(pp) => Some(pp)
+        case _                      => None
+      historyMoves += HistoryMove(move.from, move.to, castleSide, promotionPiece, pieceType, isCapture)
+      ctx = DefaultRules.applyMove(ctx, move)
+
+    val history = GameHistory(historyMoves.toList, context.halfMoveClock)
+    exportGame(headers, history)
 
   /** Export a game with headers and history to PGN format. */
   def exportGame(headers: Map[String, String], history: GameHistory): String =