Fixes #2982

Issue
When fusestatic="true" is enabled, child body inertias are incorrectly accumulated when the child body has a rotation relative to the parent. The bug was introduced in commit 7932b4b.

The problem occurs in mjCBody::AccumulateInertia where the child body's inertial orientation is not properly transformed to the parent's coordinate frame before combining inertias.

Solution
Changed line 2266 in
src/user/user_objects.cc
to use the transformed quaternion other_iquat instead of the untransformed other->iquat.

Before:

double iquat[2][4] = {
{result->iquat[0], result->iquat[1], result->iquat[2], result->iquat[3]},
{other->iquat[0], other->iquat[1], other->iquat[2], other->iquat[3]} // Untransformed
};
After:

double iquat[2][4] = {
{result->iquat[0], result->iquat[1], result->iquat[2], result->iquat[3]},
{other_iquat[0], other_iquat[1], other_iquat[2], other_iquat[3]} // Properly transformed
};