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

include/mujoco/mjvisualize.h

@@ -157,8 +157,9 @@ struct mjvPerturb_ {              // object selection and perturbation
   int      active2;               // secondary perturbation bitmask (mjtPertBit)
   mjtNum   refpos[3];             // reference position for selected object
   mjtNum   refquat[4];            // reference orientation for selected object
-  mjtNum   reflocalpos[3];        // reference selection point in object coordinates
+  mjtNum   refselpos[3];          // reference position for selection point
   mjtNum   localpos[3];           // selection point in object coordinates
+  mjtNum   localmass;             // spatial inertia at selection point
   mjtNum   scale;                 // relative mouse motion-to-space scaling (set by initPerturb)
 };
 typedef struct mjvPerturb_ mjvPerturb;