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

python/mujoco/introspect/structs.py

@@ -2673,14 +2673,6 @@ 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(