Allow flex sleeping

PiperOrigin-RevId: 917817500
Change-Id: Ia3bd5e52e7c2eaa3f70c81352d82c130b1d357f6

doc/XMLreference.rst

@@ -2113,7 +2113,7 @@ defined. Its body name is automatically defined as "world".
    - Trees which are connected by tendons which have non-zero stiffness and damping are not allowed to sleep
      (overridable).
    - Trees which are connected by tendons which connect more than two trees are not allowed to sleep (not overridable).
-   - :ref:`flexes<ElemFlex>` are not allowed to sleep (not overridable).
+   - Constraint-free :ref:`flexes<ElemFlex>` are not allowed to sleep (not overridable).
    - All other trees are allowed to sleep (overridable).
 
    The policies :at-val:`never` and :at-val:`allowed` constitute user overrides of the automatic compiler policy.

doc/changelog.rst

@@ -16,6 +16,7 @@ General
 - The pseudo-random constraint visitation order in the :ref:`PGS solver<soAlgorithms>`, introduced in the previous
   release, now uses a fixed seed. The previous implementation seeded with ``mjData.time``, which introduced subtle yet
   undesirable time dependence.
+- Flexes are now allowed to sleep, with the exception of completely passive (constraint-free) flexes.
 
 .. admonition:: Breaking API changes
    :class: attention

doc/programming/simulation.rst

@@ -1176,11 +1176,6 @@ which are initialized asleep. These can be placed in mid-air or in deep collisio
 Notes
 ^^^^^
 
-.. admonition:: Subject to change
-   :class: warning
-
-   Sleeping is a new feature (Nov 2025) that is subject to change and may have latent bugs.
-
 **Sleeping actuators**
   As explained in the :ref:`body/sleep <body-sleep>` documentation, trees with actuators are by default not allowed to
   sleep, but this can be overridden by the user. The reason sleeping is not allowed by default is that once an actuator
@@ -1269,13 +1264,6 @@ Notes
 **RK4 integrator**
   The RK4 integrator is not currently supported, due to the subtleties of waking inside the sub-steps.
 
-**Latent bugs**
-  Sleeping may have latent bugs. These bugs may generally come in two varieties:
-
-  - Quantities which could be skipped are instead recomputed. The only observable effect of such a bug would be that
-    the simulation is slower than it could be. This type of bug can only be diagnosed with detailed profiling.
-  - Actual bugs. Hopefully these will lead to informative runtime errors, please report any to the development team.
-
 
 .. _siCoordinate: