Implement bending forces for interpolated flex shells.

This change adds a new passive force computation for flexes with elastic2d="bend" and dof="trilinear". The bending energy is based on the squared difference of normals between adjacent face elements at their shared edge midpoint. The edge data is precomputed during model compilation and stored in flex_bending.

PiperOrigin-RevId: 910772638
Change-Id: I3b12c7b7f1ba6ac1875df495d89e8cfec921ca80

model/flex/flag.xml

@@ -33,7 +33,6 @@
       <flexcomp type="grid" count="9 19 1" spacing=".05 .05 .05" mass="10"
                 name="flag" radius="0.001">
         <edge equality="true" damping="0.001"/>
-        <elasticity poisson="0" thickness="1e-2" young="3e6" elastic2d="none"/>
       </flexcomp>
     </body>
   </worldbody>

model/flex/hollow_vs_solid.xml

@@ -50,7 +50,7 @@
         origin="0 0 0" count="8 2 12" cellcount="6 1 6"
         type="grid" name="soft_mesh_2" dim="3" spacing=".025 .05 .025"
         mass="0.43" radius="0.005" dof="trilinear" rgba="0.9 0.7 0.7 1">
-        <elasticity young="0" poisson="0.3" damping="0.01" elastic2d="bend" thickness="0.03"/>
+        <elasticity young="1e5" poisson="0.3" damping="0" elastic2d="bend" thickness="0.03"/>
         <edge equality="strain" solref="0.001 1" solimp="0.99 0.999 0.001"/>
         <contact selfcollide="none"
               internal="false"