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

unity/Runtime/Bindings/MjBindings.cs

@@ -2472,8 +2472,9 @@ public unsafe struct mjvPerturb_ {
   public int active2;
   public fixed double refpos[3];
   public fixed double refquat[4];
-  public fixed double reflocalpos[3];
+  public fixed double refselpos[3];
   public fixed double localpos[3];
+  public double localmass;
   public double scale;
 }