Feed move history and halfmove clock to the engine for repetition detection

- Track HalfmoveClock in GameState/ChessService (reset on captures and pawn moves)
- Emit the real halfmove clock in ToFen and add GameState.RepetitionHistory()
  (prior positions since the last irreversible move) in ChessEngineHelpers
- Pass that history through IChessEngine.GetBestMoveAsync to the native engine;
  Stockfish ignores it

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

JoshHeaps.Net/Services/Implementations/ChessService.cs

@@ -28,6 +28,7 @@ public class ChessService : IChessService
         gameState.BlackCanCastleKingside = true;
         gameState.BlackCanCastleQueenside = true;
         gameState.EnPassantTarget = null;
+        gameState.HalfmoveClock = 0;
 
         SetupBlackPieces(gameState);
         SetupWhitePieces(gameState);
@@ -178,6 +179,7 @@ public class ChessService : IChessService
     {
         var oldPos = piece.Position;
         var captured = gs.Board[targetPos.Row, targetPos.Col];
+        var isPawnMove = piece.Type == PieceType.Pawn;   // captured before promotion can change Type
 
         HandleEnPassantIfNeeded(gs, piece, targetPos, ref captured);
 
@@ -199,6 +201,9 @@ public class ChessService : IChessService
 
         UpdateCastlingRights(gs, piece, oldPos);
 
+        // Reset the 50-move clock on captures and pawn moves (irreversible); else advance it.
+        gs.HalfmoveClock = (isPawnMove || captured != null) ? 0 : gs.HalfmoveClock + 1;
+
         gs.CurrentPlayer = gs.CurrentPlayer == PieceColor.White
             ? PieceColor.Black
             : PieceColor.White;