Project out rigid body modes from flex strain equality constraints.

Modify EigendecomposeStiffness to project out rigid body translations and rotations from the stiffness matrix eigenvectors. This prevents "ghost damping" from rigid body modes being incorrectly constrained. Update hollow_vs_solid.xml with solimp parameters for edge constraints. Adjust engine_core_constraint_test.cc to reflect the reduced number of equality constraints due to the projection.

PiperOrigin-RevId: 910631207
Change-Id: Ibaaa59bc030cfc6ae657d8f0d1b2a51002cc8d4e

model/flex/hollow_vs_solid.xml

@@ -24,7 +24,7 @@
             solref="0.001 1" friction="2 0.1 0.1"/>
       <joint type="slide" axis="0 0 1" range="-1 1"/>
       <default class="finger">
-        <geom size="0.06 0.01 0.05"/>
+        <geom size="0.01 0.06 0.03"/>
         <joint type="slide" range="0 0.1"/>
       </default>
     </default>
@@ -51,7 +51,7 @@
         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"/>
-        <edge equality="strain" solref="0.001 1"/>
+        <edge equality="strain" solref="0.001 1" solimp="0.99 0.999 0.001"/>
         <contact selfcollide="none"
               internal="false"
               friction="2.0 0.5 0.01"