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/Interfaces/IChessEngine.cs

@@ -14,6 +14,11 @@ public interface IChessEngine : IAsyncDisposable
     /// Returns the engine's chosen move in UCI long-algebraic form (e.g. "e2e4", "e7e8q").
     /// </summary>
     /// <param name="fen">The current position as a FEN string.</param>
+    /// <param name="historyFens">
+    /// Prior positions since the last irreversible move, oldest first, excluding the
+    /// current one — lets the engine detect threefold/50-move draws the FEN can't carry.
+    /// May be empty.
+    /// </param>
     /// <returns>The selected move as a UCI string.</returns>
-    Task<string> GetBestMoveAsync(string fen);
+    Task<string> GetBestMoveAsync(string fen, IReadOnlyList<string> historyFens);
 }