Add implicit bending stiffness for standard flex.

Standard flex (flex_interp=0) with thin-plate bending treated bending forces purely explicitly. This caused contact-induced vertex vibrations and non-physical energy injection for flat resting sheets, because the solver treated each vertex as an independent mass during contact and contact normals are orthogonal to stretch constraints.

Fix: extend the existing preconditioned CG solver to include the constant bending stiffness K_bend in the implicit operator via matrix-free mat-vec.
PiperOrigin-RevId: 914774020
Change-Id: I45e0d6749abb6f873566203bccae956514b2576b

model/flex/poncho.xml

@@ -15,7 +15,7 @@
 
 <mujoco model="Poncho">
   <include file="mannequin.xml"/>
-  <option timestep="0.005" integrator="implicitfast" viscosity=".3" solver="CG" tolerance="1e-6">
+  <option integrator="implicitfast" solver="CG" tolerance="1e-6">
     <flag energy="enable"/>
   </option>
 
@@ -1414,7 +1414,7 @@
                 398 399 418
                 398 376 378">
       <edge equality="vert" damping="0.1"/>
-      <elasticity young="3e5" poisson="0" thickness="8e-3" elastic2d="bend"/>
+      <elasticity young="3e5" poisson="0" thickness="8e-3" elastic2d="bend" damping="0.02"/>
       <contact solref="0.003"/>
     </flexcomp>
 

model/flex/poncho_edgeequality.xml

@@ -15,7 +15,7 @@
 
 <mujoco model="Poncho">
   <include file="mannequin.xml"/>
-  <option timestep="0.005" integrator="implicitfast" viscosity=".3" solver="CG" tolerance="1e-6">
+  <option integrator="implicitfast" solver="CG" tolerance="1e-6">
     <flag energy="enable"/>
   </option>
 
@@ -1414,7 +1414,7 @@
                 398 399 418
                 398 376 378">
       <edge equality="true" damping="0.1"/>
-      <elasticity poisson="0" young="3e5" thickness="8e-3" elastic2d="bend"/>
+      <elasticity poisson="0" young="3e5" thickness="8e-3" elastic2d="bend" damping="0.02"/>
       <contact solref="0.003"/>
     </flexcomp>