Improved behaviour of mouse-spring interactions.

Also:

- Renamed recently introduced `mjvPerturb` member `reflocalpos` -> `refselpos` and corrected docstring.
- Added `mjvPerturb.localmass`: body spatial inertia at selection point.
- Removed `const` qualifier to `mjData` argument of `mjv_initPerturb()`, to facilitate spatial inertia computation.
- Improved `mj_Object{Velocity, Acceleration}` docstrings, clarifying that rotational velocities precede linear velocities.
- Removed redundant const qualifiers from non pointer arguments to `mju_dot` and `mju_writeNumBytes`.

PiperOrigin-RevId: 511002168
Change-Id: I27400f497947f330072d4abb7c9bf7e71e6c5d49

src/engine/engine_vis_interact.h

@@ -59,8 +59,7 @@ MJAPI void mjv_moveModel(const mjModel* m, int action, mjtNum reldx, mjtNum reld
                          const mjtNum roomup[3], mjvScene* scn);
 
 // copy perturb pos,quat from selected body; set scale perturbation
-MJAPI void mjv_initPerturb(const mjModel* m, const mjData* d,
-                           const mjvScene* scn, mjvPerturb* pert);
+MJAPI void mjv_initPerturb(const mjModel* m, mjData* d, const mjvScene* scn, mjvPerturb* pert);
 
 // set perturb pos,quat in d->mocap when selected body is mocap, and in d->qpos otherwise
 //  d->qpos written only if flg_paused and subtree root for selected body has free joint