Correct DC motor derivative calculation and enforce actearly.

PiperOrigin-RevId: 898983657
Change-Id: I008529eacf3e400696d26bd3a52a4cf4c1c12225

src/user/user_api.cc

@@ -1280,6 +1280,9 @@ const char* mjs_setToDCMotor(mjsActuator* actuator, double motorconst[2], double
   // enforce actlimited = 0; homogeneous bounds are invalid across DC motor states
   actuator->actlimited = 0;
 
+  // DC motor always uses actearly
+  actuator->actearly = 1;
+
   return "";
 }