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

src/engine/engine_core_constraint.h

@@ -18,6 +18,7 @@
 #include <mujoco/mjdata.h>
 #include <mujoco/mjexport.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mjtnum.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -35,6 +36,9 @@ MJAPI void mj_mulJacVec(const mjModel* m, const mjData* d, mjtNum* res, const mj
 // multiply JacobianT by vector
 MJAPI void mj_mulJacTVec(const mjModel* m, const mjData* d, mjtNum* res, const mjtNum* vec);
 
+// subtract Jdot*v correction from result vector
+MJAPI void mj_Jdotv(const mjModel* m, mjData* d, mjtNum* result);
+
 
 //-------------------------- utility functions -----------------------------------------------------