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

model/car/car.xml

@@ -0,0 +1,72 @@
+<mujoco>
+  <compiler autolimits="true"/>
+
+  <asset>
+    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
+    <material name="grid" texture="grid" texrepeat="1 1" texuniform="true" reflectance=".2"/>
+    <mesh name="chasis" scale=".01 .006 .0015"
+      vertex=" 9   2   0
+              -10  10  10
+               9  -2   0
+               10  3  -10
+               10 -3  -10
+              -8   10 -10
+              -10 -10  10
+              -8  -10 -10
+              -5   0   20"/>
+  </asset>
+
+  <default>
+    <joint damping=".03" actuatorforcerange="-0.5 0.5"/>
+    <default class="wheel">
+      <geom type="cylinder" size=".03 .01" rgba=".5 .5 1 1"/>
+    </default>
+    <default class="decor">
+      <site type="box" rgba=".5 1 .5 1"/>
+    </default>
+  </default>
+
+  <worldbody>
+    <geom type="plane" size="3 3 .01" material="grid"/>
+    <body name="car" pos="0 0 .03">
+      <freejoint/>
+      <light name="top light" pos="0 0 2" mode="trackcom" diffuse=".4 .4 .4"/>
+      <geom name="chasis" type="mesh" mesh="chasis"/>
+      <geom name="front wheel" pos=".08 0 -.015" type="sphere" size=".015" condim="1" priority="1"/>
+      <light name="front light" pos=".1 0 .02" dir="2 0 -1" diffuse="1 1 1"/>
+      <body name="left wheel" pos="-.07 .06 0" zaxis="0 1 0">
+        <joint name="left"/>
+        <geom class="wheel"/>
+        <site class="decor" size=".006 .025 .012"/>
+        <site class="decor" size=".025 .006 .012"/>
+      </body>
+      <body name="right wheel" pos="-.07 -.06 0" zaxis="0 1 0">
+        <joint name="right"/>
+        <geom class="wheel"/>
+        <site class="decor" size=".006 .025 .012"/>
+        <site class="decor" size=".025 .006 .012"/>
+      </body>
+    </body>
+  </worldbody>
+
+  <tendon>
+    <fixed name="forward">
+      <joint joint="left" coef=".5"/>
+      <joint joint="right" coef=".5"/>
+    </fixed>
+    <fixed name="turn">
+      <joint joint="left" coef="-.5"/>
+      <joint joint="right" coef=".5"/>
+    </fixed>
+  </tendon>
+
+  <actuator>
+    <motor name="forward" tendon="forward" ctrlrange="-1 1"/>
+    <motor name="turn" tendon="turn" ctrlrange="-1 1"/>
+  </actuator>
+
+  <sensor>
+    <jointactuatorfrc name="right" joint="right"/>
+    <jointactuatorfrc name="left" joint="left"/>
+  </sensor>
+</mujoco>