Use banded solver for implicit flex integration.

The flex interpolation stiffness matrix within the implicit/implicitfast solvers is now built and factorized in a banded format instead of a dense one. This involves:
-   Calculating the bandwidth based on the sparsity of the mass/damping matrix and the connectivity within flex cells.
-   Allocating and populating a banded matrix `H`.
-   Using `mju_cholFactorBand` and `mju_cholSolveBand` for factorization and solving.
This change improves performance for flexes with many DOFs but local coupling.

PiperOrigin-RevId: 901297952
Change-Id: I3efe06353d1903ea65ab30dc49685cede228bb68

src/user/user_mesh.cc

@@ -4095,6 +4095,9 @@ void mjCFlex::Compile(const mjVFS* vfs) {
 
   // check node compatibility with count and dof
   if (spec.order > 0) {
+    if (spec.cellcount[0] == 0 || spec.cellcount[1] == 0 || spec.cellcount[2] == 0) {
+      throw mjCError(this, "cellcount cannot be 0 in any dimension when interpolation order > 0");
+    }
     int expected_nodes = (spec.cellcount[0] * spec.order + 1) *
                          (spec.cellcount[1] * spec.order + 1) *
                          (spec.cellcount[2] * spec.order + 1);