Add Jdot correction for connect and weld constaints

Measured reduction in constraint violations before/after this change:

| Model | Correction ON (Avg Viol) | Correction OFF (Avg Viol) | Reduction |
| :--- | :--- | :--- | :--- |
| `jdotv_connect_2d.xml` | 3.959e-4 | 1.699e-3 | **76.7%** |
| `jdotv_connect_3d.xml` | 1.399e-3 | 5.493e-3 | **74.5%** |
| `jdotv_weld_3d.xml` | 9.472e-3 | 1.148e-2 | **17.5%** |

PiperOrigin-RevId: 899137525
Change-Id: Ic3e33764ebd64239bab916289c23d80c3da0b51b

doc/changelog.rst

@@ -29,9 +29,13 @@ General
   may unify the linear and higher-order coefficients into a single array.
 - Added :ref:`midpoint integration<geMidpoint>` for standalone free bodies in ``implicit`` and ``implicitfast``
   :ref:`integrators<geIntegrators>`. This applies the implicit midpoint rule to the rotational dynamics of free bodies
-  with no children, exactly conserving kinetic energy and angular momentum in the absence of external torques. The
+  with no children, conserving kinetic energy to machine precision in the absence of external torques. The
   :ref:`invdiscrete<option-flag-invdiscrete>` flag now also disables midpoint integration, providing an opt-out
   mechanism.
+- Added the centripetal/Coriolis acceleration term :math:`\dot{J}v` to the constraint solver bias for
+  :ref:`connect<equality-connect>` and :ref:`weld<equality-weld>` equality constaints. This significantly improves the
+  stability of constrained mechanisms like four-bar linkages. See :ref:`Dual problem<soDual>` for details.
+
 - Introduced :ref:`mjpEncoder`, the counterpart to :ref:`mjpDecoder` for encoding of :ref:`mjSpec` and :ref:`mjModel`
   into :ref:`mjResource`.