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/engine/engine_sensor.c

@@ -593,6 +593,7 @@ void mj_sensorAcc(const mjModel* m, mjData* d) {
       if (rnePost == 0                  &&
           type != mjSENS_TOUCH          &&
           type != mjSENS_ACTUATORFRC    &&
+          type != mjSENS_JOINTACTFRC    &&
           type != mjSENS_JOINTLIMITFRC  &&
           type != mjSENS_TENDONLIMITFRC) {
         // compute cacc, cfrc_int, cfrc_ext
@@ -686,6 +687,10 @@ void mj_sensorAcc(const mjModel* m, mjData* d) {
         d->sensordata[adr] = d->actuator_force[objid];
         break;
 
+      case mjSENS_JOINTACTFRC:                            // jointactfrc
+        d->sensordata[adr] = d->qfrc_actuator[m->jnt_dofadr[objid]];
+        break;
+
       case mjSENS_JOINTLIMITFRC:                          // jointlimitfrc
         d->sensordata[adr] = 0;
         for (int j=ne+nf; j < nefc; j++) {