fix: Don't use GateTranslator.xor

No need to have an "exactly-one" of piece rotations in
Constraints.addExactlyOnePiecePerPositionTo:
Constraints.addExactlyOnePositionPerPieceTo already
constrains it.

Plus GateTranslator.xor is not an "exactly-one", it is
"parity" xor so it is more confusing than anything.

src/main/java/re/belv/eternity2/solver/Constraints.java

@@ -64,7 +64,7 @@ void addExactlyOnePiecePerPositionTo(final ISolver solver) throws ContradictionE
                         positionPieceRotations.push(variables.representingPiece(rowIndex, columnIndex, pieceIndex, rotation));
                     }
                     final int positionPiece = solver.nextFreeVarId(true);
-                    gator.xor(positionPiece, positionPieceRotations);
+                    gator.or(positionPiece, positionPieceRotations);
                     positionPieces.push(positionPiece);
                 }
                 solver.addExactly(positionPieces, 1);