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/Models/GameState.cs

@@ -40,6 +40,10 @@ public class GameState
     // Position keys (FEN placement/side/castling/en-passant) for threefold-repetition detection.
     public List<string> PositionHistory { get; set; }
 
+    // Half-moves since the last capture or pawn move (the FEN 50-move clock). Also tells
+    // us how many trailing PositionHistory entries belong to the current repetition window.
+    public int HalfmoveClock { get; set; }
+
     // A list of all pieces to quickly reference them (optional but convenient).
     // Alternatively, you can iterate the Board array.
     public List<ChessPiece> Pieces { get; set; }