Add internal functionmj_tendonDot: time derivative of tendon Jacobian for one tendon.

Notes:
- Currently always uses dense math, even for sparse models. This should be easy to change in the future.
- Does not support geom wrapping. This is possible but harder, requires derivatives of mju_wrap.
PiperOrigin-RevId: 740378741
Change-Id: Id39ef2c4bfbb7ee11ec33c97d7d83140441cdab2

src/engine/engine_core_smooth.h

@@ -39,6 +39,9 @@ MJAPI void mj_flex(const mjModel* m, mjData* d);
 // compute tendon lengths, velocities and moment arms
 MJAPI void mj_tendon(const mjModel* m, mjData* d);
 
+// compute time derivative of dense tendon Jacobian for one tendon
+MJAPI void mj_tendonDot(const mjModel* m, mjData* d, int id, mjtNum* Jdot);
+
 // compute actuator transmission lengths and moments
 MJAPI void mj_transmission(const mjModel* m, mjData* d);