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_support.c

@@ -885,7 +885,7 @@ void mju_camIntrinsics(const mjModel* m, int camid,
 // read delayed ctrl value for actuator at given time
 mjtNum mj_readCtrl(const mjModel* m, const mjData* d, int id, mjtNum time, int interp) {
   // validate actuator id
-  if (id < 0 || id >= m->nu) {
+  if (id < 0 || id >= m->nactuator) {
     mjERROR("invalid actuator id %d", id);
     return 0;
   }
@@ -893,7 +893,7 @@ mjtNum mj_readCtrl(const mjModel* m, const mjData* d, int id, mjtNum time, int i
   // no delay: return current ctrl value
   int nsample = m->actuator_history[2*id];
   if (nsample == 0) {
-    return d->ctrl[id];
+    return d->ctrl[m->actuator_ctrladr[id]];
   }
 
   // resolve interpolation order: use model's interp if argument is -1
@@ -938,7 +938,7 @@ const mjtNum* mj_readSensor(const mjModel* m, const mjData* d, int id, mjtNum ti
 void mj_initCtrlHistory(const mjModel* m, mjData* d, int id,
                         const mjtNum* times, const mjtNum* values) {
   // validate actuator id
-  if (id < 0 || id >= m->nu) {
+  if (id < 0 || id >= m->nactuator) {
     mjERROR("invalid actuator id %d", id);
     return;
   }