Replace the banded Cholesky solver for implicit flex interpolation

with a preconditioned Conjugate Gradient (CG) solver that operates
on the full system matrix.

The previous approach extracted flex DOFs into a reduced banded system,
factored it separately, and overwrote the global solve. This required
precomputed bandwidth (makeFlexBandwidth), parent-joint detection,
coupling corrections, and a FlexInterpContext struct — and only worked
for standalone flex trees without parent joints.

The new CG solver uses the already-factored global system (M - h*qDeriv)
as a preconditioner and adds the flex stiffness contribution via
matrix-free products (mjd_flexInterp_mulKD/mulK). This handles any
kinematic configuration — including flexes attached to articulated
chains or with parent joints — without sparsity pattern restrictions.

Before (`bunny_multicell`):
```
 Simulation time      : 50.80 s
 Steps per second     : 197
 Realtime factor      : 0.20 x
 Time per step        : 5080.3 µs

 CG iters / step      : 3.16
 Contacts / step      : 31.04
 Constraints / step   : 124.15
 Degrees of freedom   : 178
 Dynamic memory usage : 0.4% of 100M
```

After:
```
 Simulation time      : 9.52 s
 Steps per second     : 1051
 Realtime factor      : 1.05 x
 Time per step        : 951.7 µs

 CG iters / step      : 3.21
 Contacts / step      : 30.90
 Constraints / step   : 123.61
 Degrees of freedom   : 178
 Dynamic memory usage : 0.3% of 100M
```

PiperOrigin-RevId: 913758038
Change-Id: If5aa617b2d535c86aec9bd71c9e0003a2b38bdd7

wasm/codegen/generated/bindings.cc

@@ -4655,9 +4655,6 @@ 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));
   }
@@ -11854,7 +11851,6 @@ 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_bendingadr", &MjModel::flex_bendingadr)
     .property("flex_bvhadr", &MjModel::flex_bvhadr)