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_util_blas.c

@@ -627,7 +627,7 @@ mjtNum mju_norm(const mjtNum* res, int n) {
 
 
 // vector dot-product
-mjtNum mju_dot(const mjtNum* vec1, const mjtNum* vec2, const int n) {
+mjtNum mju_dot(const mjtNum* vec1, const mjtNum* vec2, int n) {
   mjtNum res = 0;
   int i = 0;
   int n_4 = n - 4;