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

wasm/codegen/generated/bindings.cc

@@ -4649,6 +4649,9 @@ struct MjModel {
   emscripten::val flex_interp() const {
     return emscripten::val(emscripten::typed_memory_view(ptr_->nflex, ptr_->flex_interp));
   }
+  emscripten::val flex_bandwidth() const {
+    return emscripten::val(emscripten::typed_memory_view(ptr_->nflex, ptr_->flex_bandwidth));
+  }
   emscripten::val flex_cellnum() const {
     return emscripten::val(emscripten::typed_memory_view(ptr_->nflex * 3, ptr_->flex_cellnum));
   }
@@ -11815,6 +11818,7 @@ EMSCRIPTEN_BINDINGS(mujoco_bindings) {
     .property("eq_type", &MjModel::eq_type)
     .property("exclude_signature", &MjModel::exclude_signature)
     .property("flex_activelayers", &MjModel::flex_activelayers)
+    .property("flex_bandwidth", &MjModel::flex_bandwidth)
     .property("flex_bending", &MjModel::flex_bending)
     .property("flex_bvhadr", &MjModel::flex_bvhadr)
     .property("flex_bvhnum", &MjModel::flex_bvhnum)