Changes to dcmotor:

- Remove `lugre:viscous`, should now be added directly to actuator `damping`. Trying to do this for the user was incompatible with default inheritance (compounding instead of overriding).
- Move voltage limiting from the `saturation` to the `controller` attribute.
- Fix indexing issues in default inheritance.

PiperOrigin-RevId: 897087642
Change-Id: I5388c2633e15c7e223992e7eb5d6a28db75a6438

test/engine/testdata/derivative/dcmotor.xml

@@ -30,6 +30,6 @@
 
     <!-- DC motor with LuGre friction (sigma1 micro-damping) -->
     <dcmotor name="dc_lugre" joint="joint4" motorconst="0.05" resistance="2.0"
-             lugre="1e4 100 0.001 0.005 0.008 0.1"/>
+             damping="0.001" lugre="1e4 100 0.005 0.008 0.1"/>
   </actuator>
 </mujoco>