feat: add FEN exporter and round-trip tests

Implements FenExporter.boardToFen() converting Board to FEN piece-placement string,
and adds three round-trip tests (initial position, empty board, partial position).

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

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

@@ -1,8 +1,11 @@
 package de.nowchess.chess.notation
 
 import de.nowchess.api.board.*
+<<<<<<< HEAD
 import de.nowchess.api.game.{CastlingRights, GameState}
 import de.nowchess.api.board.Color
+=======
+>>>>>>> cc62cd2 (feat: add FEN exporter and round-trip tests)
 
 object FenExporter:
 
@@ -31,6 +34,7 @@ object FenExporter:
     if emptyCount > 0 then rankChars += emptyCount.toString.charAt(0)
     rankChars.mkString
 
+<<<<<<< HEAD
   /** Convert a GameState to a complete FEN string. */
   def gameStateToFen(state: GameState): String =
     val piecePlacement = state.piecePlacement
@@ -48,6 +52,8 @@ object FenExporter:
     val result = s"$wk$wq$bk$bq"
     if result.isEmpty then "-" else result
 
+=======
+>>>>>>> cc62cd2 (feat: add FEN exporter and round-trip tests)
   /** Convert a Piece to its FEN character (uppercase = White, lowercase = Black). */
   private def pieceToPgnChar(piece: Piece): Char =
     val base = piece.pieceType match

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

@@ -40,3 +40,24 @@ class FenParserTest extends AnyFunSuite with Matchers:
 
     board.map(_.pieceAt(Square(File.E, Rank.R6))) shouldBe Some(Some(Piece.BlackKing))
     board.map(_.pieceAt(Square(File.E, Rank.R4))) shouldBe Some(Some(Piece.WhiteKing))
+
+  test("testRoundTripInitialPosition"):
+    val originalFen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)
+
+  test("testRoundTripEmptyBoard"):
+    val originalFen = "8/8/8/8/8/8/8/8"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)
+
+  test("testRoundTripPartialPosition"):
+    val originalFen = "8/8/4k3/8/4K3/8/8/8"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)