wip: don't lock command mutex for each mass/spring in FillGraph()

include/physics.hpp

@@ -188,6 +188,10 @@ public:
   auto AddSpringCmd(const State &a, const State &b) -> void;
 
   auto ClearCmd() -> void;
+
+  auto NonLockingAddMassCmd(const State &_state) -> void;
+
+  auto NonLockingAddSpringCmd(const State &a, const State &b) -> void;
 };
 
 // https://en.cppreference.com/w/cpp/utility/variant/visit