Introduces a new "strain" equality mode (flex_edgeequality = 3) for flexcomps with trilinear interpolation. This mode enforces constraints on the three invariants of the Green-Lagrange strain tensor at 8 Gauss quadrature points within each flex element, resulting in 24 equality constraints per flexcomp.

Also fixes a bug when attaching a flex with constraints.

PiperOrigin-RevId: 881349765
Change-Id: I8e6df8239488bfdc87d7e5b473568975e38c737e

src/user/user_mesh.cc

@@ -3876,6 +3876,7 @@ void mjCFlex::PointToLocal() {
 
 
 void mjCFlex::NameSpace(const mjCModel* m) {
+  mjCBase::NameSpace(m);
   for (auto& name : spec_vertbody_) {
     name = m->prefix + name + m->suffix;
   }