Don't trash worklist initial contents

ConflictSet.cpp

@@ -401,7 +401,8 @@ void runInterleaved(std::span<Stepwise> remaining, int stepLimit = -1) {
       bool done = remaining[i].step();
       if (done) {
         if (i != int(remaining.size()) - 1) {
-          remaining[i] = remaining.back();
+          using std::swap;
+          swap(remaining[i], remaining.back());
         }
         remaining = remaining.subspan(0, remaining.size() - 1);
       } else {