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_model.cc

@@ -3533,6 +3533,10 @@ void mjCModel::CopyObjects(mjModel* m) {
           m->flex_edgeequality[i] = 2;
           break;
         }
+        if (equalities_[k]->type == mjEQ_FLEXSTRAIN) {
+          m->flex_edgeequality[i] = 3;
+          break;
+        }
       }
     }