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

doc/changelog.rst

@@ -31,6 +31,11 @@ General
   error instead of the generic "could not decode content" message.
 - Added support for resource writing via :ref:`mju_writeResource` and the ``write`` callback in :ref:`mjpResourceProvider`.
 - Added support for :ref:`multiccd <coMultiCCD>` with arbitrarily large meshes.
+- Refactored actuator infrastructure in preparation for MIMO (multi-input multi-output) actuator support. Each actuator
+  now has ``ctrlnum`` (number of controls) and ``outnum`` (number of force outputs). The total counts
+  ``nu = sum(ctrlnum)`` and ``nout = sum(outnum)`` dimension ``mjData.ctrl`` and ``mjData.actuator_force``,
+  respectively, ``nactuator`` is the number of actuators. For existing actuators ``ctrnum = outnum = 1``, so
+  ``nactuator == nu == nout`` and existing code is unaffected.
 
 .. admonition:: Breaking API changes
    :class: attention