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/engine/engine_io.c

@@ -1958,6 +1958,7 @@ const char* mj_validateReferences(const mjModel* m) {
 
     case mjEQ_FLEX:
     case mjEQ_FLEXVERT:
+    case mjEQ_FLEXSTRAIN:
       if (obj1id >= m->nflex || obj1id < 0) {
         return "Invalid model: eq_obj1id out of bounds.";
       }