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

python/mujoco/introspect/enums.py

@@ -222,7 +222,8 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('mjEQ_TENDON', 3),
              ('mjEQ_FLEX', 4),
              ('mjEQ_FLEXVERT', 5),
-             ('mjEQ_DISTANCE', 6),
+             ('mjEQ_FLEXSTRAIN', 6),
+             ('mjEQ_DISTANCE', 7),
          ]),
      )),
     ('mjtWrap',