Refactor flex stiffness storage to support variable sizes.

This change introduces `nflexstiffness` and `flex_stiffnessadr` to allow flex stiffness matrices to have sizes other than the fixed 21 per element. Higher-order flexes can now store larger stiffness matrices based on their number of nodes. The `flex_stiffnessadr` array provides the starting index for each flex's stiffness data within the `flex_stiffness` array.

PiperOrigin-RevId: 899632263
Change-Id: Ie49182c46c3777acf0c6b492345edfcf62fb5e44

src/user/user_mesh.cc

@@ -4271,12 +4271,8 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     }
 
     // linear elasticity
-    stiffness.assign(21*nelem, 0);
-    if (interpolated) {
-      int min_size = ceil(nodexpos.size()*nodexpos.size() / 21);
-      if (min_size > nelem) {
-        throw mjCError(this, "Trilinear dofs are require at least %d elements", "", min_size);
-      }
+    if (!interpolated) {
+      stiffness.assign(21 * nelem, 0);
     }
 
     // geometrically nonlinear elasticity
@@ -4333,6 +4329,11 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   if (!stiffness_cached && young > 0 && interpolated) {
+    int n = pow(order_ + 1, 3);
+    int ndof = 3 * n;
+    if (stiffness.size() < ndof * ndof) {
+      stiffness.resize(ndof * ndof, 0);
+    }
     ComputeLinearStiffness(stiffness, nodexpos.data(), young, poisson, order_);
   }