Add 2D membrane elasticity for interpolated flex shell mode

When elastic2d="stretch" is set on an interpolated flexcomp, treat the bounding box boundary as membrane elements rather than volumetric cells. This computes plane-stress stiffness over the boundary faces and updates the runtime force/derivative kernels accordingly.

Interior vertex tracking (moving vertices that follow the deforming shell) is not yet implemented so all mesh vertices need to be on the bounding box surface or the background grid should have no interior nodes (i.e. cellcount should be 1 on at least one axis).

PiperOrigin-RevId: 907654080
Change-Id: I51b90e2f6a1d1b036f9604e42de20e377dc5d3f9

src/engine/engine_util_misc.h

@@ -103,6 +103,19 @@ MJAPI void mju_flexGatherCellState(int order, int cy, int cz, int ci, int cj, in
                                    const mjtNum* xpos0_g, mjtNum* xpos_c, mjtNum* vel_c,
                                    mjtNum* xpos0_c, int* nodeindices, mjtNum* quat);
 
+// gather face-element-local quantities and optionally compute rotation (shell mode)
+MJAPI void mju_flexGatherFaceState(int order, int cx, int cy, int cz,
+                                   int face_elem_idx,
+                                   const mjtNum* xpos_g, const mjtNum* vel_g,
+                                   const mjtNum* xpos0_g,
+                                   mjtNum* xpos_f, mjtNum* vel_f, mjtNum* xpos0_f,
+                                   int* nodeindices, mjtNum* quat);
+
+// compute corotational rotation from 2D deformation gradient on a flat face
+MJAPI void mju_flexInterpRotation2D(int order, const mjtNum* xpos_f, int npe,
+                                    int axis0, int axis1, int normal_axis,
+                                    const mjtNum local[2], mjtNum* quat);
+
 
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