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/xml/xml_native_writer.cc

@@ -711,6 +711,7 @@ void mjXWriter::OneEquality(XMLElement* elem, const mjCEquality* equality, mjCDe
 
       case mjEQ_FLEX:
       case mjEQ_FLEXVERT:
+      case mjEQ_FLEXSTRAIN:
         WriteAttrTxt(elem, "flex", mjs_getString(equality->name1));
         break;