fix(physics): the external program messages (multiplayer) no longer keep the physics steps in one thread, only network sessions do; the debug panel tells what keeps them in one

utilities/Timer.h

@@ -204,6 +204,8 @@ struct subsystem_stopwatches {
         // the steps of the physics: threads, the groups of vehicles they were split in, the largest one, the groups done in the main
         // thread (vehicle_table::update_steps_parallel())
         unsigned int threads { 1 };
+        unsigned int pool { 1 }; // threads of worker_pool
+        std::string singlethread; // what keeps the steps in one thread, if anything
         unsigned int groups { 0 };
         unsigned int largestgroup { 0 };
         unsigned int maingroups { 0 };