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

unity/Runtime/Bindings/MjBindings.cs

@@ -325,29 +325,30 @@ public enum mjtSensor : int{
   mjSENS_ACTUATORPOS = 12,
   mjSENS_ACTUATORVEL = 13,
   mjSENS_ACTUATORFRC = 14,
-  mjSENS_BALLQUAT = 15,
-  mjSENS_BALLANGVEL = 16,
-  mjSENS_JOINTLIMITPOS = 17,
-  mjSENS_JOINTLIMITVEL = 18,
-  mjSENS_JOINTLIMITFRC = 19,
-  mjSENS_TENDONLIMITPOS = 20,
-  mjSENS_TENDONLIMITVEL = 21,
-  mjSENS_TENDONLIMITFRC = 22,
-  mjSENS_FRAMEPOS = 23,
-  mjSENS_FRAMEQUAT = 24,
-  mjSENS_FRAMEXAXIS = 25,
-  mjSENS_FRAMEYAXIS = 26,
-  mjSENS_FRAMEZAXIS = 27,
-  mjSENS_FRAMELINVEL = 28,
-  mjSENS_FRAMEANGVEL = 29,
-  mjSENS_FRAMELINACC = 30,
-  mjSENS_FRAMEANGACC = 31,
-  mjSENS_SUBTREECOM = 32,
-  mjSENS_SUBTREELINVEL = 33,
-  mjSENS_SUBTREEANGMOM = 34,
-  mjSENS_CLOCK = 35,
-  mjSENS_PLUGIN = 36,
-  mjSENS_USER = 37,
+  mjSENS_JOINTACTFRC = 15,
+  mjSENS_BALLQUAT = 16,
+  mjSENS_BALLANGVEL = 17,
+  mjSENS_JOINTLIMITPOS = 18,
+  mjSENS_JOINTLIMITVEL = 19,
+  mjSENS_JOINTLIMITFRC = 20,
+  mjSENS_TENDONLIMITPOS = 21,
+  mjSENS_TENDONLIMITVEL = 22,
+  mjSENS_TENDONLIMITFRC = 23,
+  mjSENS_FRAMEPOS = 24,
+  mjSENS_FRAMEQUAT = 25,
+  mjSENS_FRAMEXAXIS = 26,
+  mjSENS_FRAMEYAXIS = 27,
+  mjSENS_FRAMEZAXIS = 28,
+  mjSENS_FRAMELINVEL = 29,
+  mjSENS_FRAMEANGVEL = 30,
+  mjSENS_FRAMELINACC = 31,
+  mjSENS_FRAMEANGACC = 32,
+  mjSENS_SUBTREECOM = 33,
+  mjSENS_SUBTREELINVEL = 34,
+  mjSENS_SUBTREEANGMOM = 35,
+  mjSENS_CLOCK = 36,
+  mjSENS_PLUGIN = 37,
+  mjSENS_USER = 38,
 }
 public enum mjtStage : int{
   mjSTAGE_NONE = 0,
@@ -1987,12 +1988,14 @@ public unsafe struct mjModel_ {
   public int* jnt_bodyid;
   public int* jnt_group;
   public byte* jnt_limited;
+  public byte* jnt_actfrclimited;
   public double* jnt_solref;
   public double* jnt_solimp;
   public double* jnt_pos;
   public double* jnt_axis;
   public double* jnt_stiffness;
   public double* jnt_range;
+  public double* jnt_actfrcrange;
   public double* jnt_margin;
   public double* jnt_user;
   public int* dof_bodyid;