Make constraint island discovery on by default.

Also fix latent bug in mjData serialization.

PiperOrigin-RevId: 799505349
Change-Id: I299e7cc8133fa2ec0b339a7ff3d02543e06778b1

doc/XMLreference.rst

@@ -587,6 +587,14 @@ from its default.
    This flag enables the native convex collision detection pipeline instead of using the
    `libccd library <https://github.com/danfis/libccd>`__, see :ref:`convex collisions<coCCD>` for more details.
 
+.. _option-flag-island:
+
+:at:`island`: :at-val:`[disable, enable], "enable"`
+   This flag enables discovery and construction of constraint islands: disjoint sets of constraints and
+   degrees-of-freedom that do not interact and can be solved independently. Islanding is not yet supported by the PGS
+   solver. See :ref:`soIsland` for more details. The :ref:`mjVIS_ISLAND <mjtVisFlag>` enables
+   `island visualization <https://youtu.be/Vc1tq0fFvQA>`__.
+
 .. _option-flag-eulerdamp:
 
 :at:`eulerdamp`: :at-val:`[disable, enable], "enable"`
@@ -647,14 +655,6 @@ from its default.
    to instabilities that typically manifest as sliding or wobbling. The implementation of this feature depends on the
    selected convex collision pipeline, see :ref:`convex collisions<coCCD>` for more details.
 
-.. _option-flag-island:
-
-:at:`island`: :at-val:`[disable, enable], "disable"`
-   This flag enables discovery of constraint islands: disjoint sets of constraints and
-   degrees-of-freedom that do not interact. The flag currently has no effect on the physics pipeline, but enabling it
-   allows for `island visualization <https://youtu.be/Vc1tq0fFvQA>`__.
-   In a future release, the constraint solver will exploit the disjoint nature of constraint islands.
-
 .. _compiler:
 
 **compiler** (*)