Improvements related to models where joint-actuator relationship is not one-to-one:

- Add `joint-actuatorforcerange` for clamping total actuator force at joints. Add `sensor-jointactuatorfrc` for sensing total actuator forces on a single joint.
See [documentation](https://mujoco.readthedocs.io/en/latest//modeling.html#actuator-force-clamping) for justification and use cases.
- Add simple car model to `model/`.
- Move actuation-related test models into `engine/testdata/actuation/`.

PiperOrigin-RevId: 549355941
Change-Id: I27f6c1f80426d73a2811ef5ae74684a228b2fbd1

src/user/user_model.cc

@@ -1407,6 +1407,7 @@ void mjCModel::CopyTree(mjModel* m) {
       m->jnt_type[jid] = pj->type;
       m->jnt_group[jid] = pj->group;
       m->jnt_limited[jid] = pj->limited;
+      m->jnt_actfrclimited[jid] = pj->actfrclimited;
       m->jnt_qposadr[jid] = qposadr;
       m->jnt_dofadr[jid] = dofadr;
       m->jnt_bodyid[jid] = pj->body->id;
@@ -1414,6 +1415,7 @@ void mjCModel::CopyTree(mjModel* m) {
       copyvec(m->jnt_axis+3*jid, pj->locaxis, 3);
       m->jnt_stiffness[jid] = (mjtNum)pj->stiffness;
       copyvec(m->jnt_range+2*jid, pj->range, 2);
+      copyvec(m->jnt_actfrcrange+2*jid, pj->actfrcrange, 2);
       copyvec(m->jnt_solref+mjNREF*jid, pj->solref_limit, mjNREF);
       copyvec(m->jnt_solimp+mjNIMP*jid, pj->solimp_limit, mjNIMP);
       m->jnt_margin[jid] = (mjtNum)pj->margin;