Fix implicit integrator derivatives for actearly actuators.

The derivative calculation for actuator velocity in implicit integrators now correctly accounts for the `actearly` flag, using the next activation value when `actearly` is true.

PiperOrigin-RevId: 868598722
Change-Id: Ia180afb15b31a718170aeaf9d4ac514bb9e6073b

src/engine/engine_support.h

@@ -106,6 +106,10 @@ MJAPI void mj_normalizeQuat(const mjModel* m, mjtNum* qpos);
 // return 1 if actuator i is disabled, 0 otherwise
 MJAPI int mj_actuatorDisabled(const mjModel* m, int i);
 
+// returns the next activation given current act_dot, after clamping
+mjtNum mj_nextActivation(const mjModel* m, const mjData* d,
+                         int actuator_id, int act_adr, mjtNum act_dot);
+
 // sum all body masses
 MJAPI mjtNum mj_getTotalmass(const mjModel* m);