Preparation for MIMO actuators: split actuator counts: nu (inputs), nactuator (objects), nout (outputs).

An actuator now owns a block of consecutive controls
(actuator_ctrladr/ctrlnum, width defined by the actuator type) and a block
of consecutive force outputs (actuator_outadr/outnum, width defined by the
transmission type). Force outputs are the scalars of actuation space: one
force, length, velocity and moment row each. nout = dim(actuator_force) is
derived from transmission types; all current types have width 1, so all
three counts coincide for every existing model and behavior is bit-exact.

Array re-keying: ctrlrange/ctrllimited by nu; forcerange/forcelimited/gear/
acc0/length0/lengthrange and the moment row structure by nout; everything
else per actuator. The mjModel actuator block is re-sorted by size key.

Layout-breaking, not behavior-breaking: saved .mjb files are invalidated
(size list changed) and recompilation is required.

PiperOrigin-RevId: 948351772
Change-Id: Icbc196ffa083cb1eaa6f1a3710869c89d8f62540

src/engine/engine_name.c

@@ -160,10 +160,10 @@ static int _getnumadr(const mjModel* m, mjtObj type, int** padr, int* mapadr) {
     mjFALLTHROUGH;
 
   case mjOBJ_ACTUATOR:
-    *mapadr -= mjLOAD_MULTIPLE*m->nu;
+    *mapadr -= mjLOAD_MULTIPLE*m->nactuator;
     if (num < 0) {
       *padr = m->name_actuatoradr;
-      num = m->nu;
+      num = m->nactuator;
     }
     mjFALLTHROUGH;