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

python/mujoco/introspect/structs.py

@@ -2668,6 +2668,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='interpolation (0: vertex, 1: nodes)',
                  array_extent=('nflex',),
              ),
+             StructFieldDecl(
+                 name='flex_bandwidth',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='precomputed solver bandwidth',
+                 array_extent=('nflex',),
+             ),
              StructFieldDecl(
                  name='flex_cellnum',
                  type=PointerType(