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

introspect/enums.py

@@ -315,29 +315,30 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('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),
          ]),
      )),
     ('mjtStage',

introspect/structs.py

@@ -1391,6 +1391,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='does joint have limits                   (njnt x 1)',
              ),
+             StructFieldDecl(
+                 name='jnt_actfrclimited',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtByte'),
+                 ),
+                 doc='does joint have actuator force limits    (njnt x 1)',
+             ),
              StructFieldDecl(
                  name='jnt_solref',
                  type=PointerType(
@@ -1433,6 +1440,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='joint limits                             (njnt x 2)',
              ),
+             StructFieldDecl(
+                 name='jnt_actfrcrange',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+                 doc='range of total actuator force            (njnt x 2)',
+             ),
              StructFieldDecl(
                  name='jnt_margin',
                  type=PointerType(