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

include/mujoco/mjmodel.h

@@ -979,7 +979,6 @@ struct mjModel_ {
   int*      flex_matid;           // material id for rendering                (nflex x 1)
   int*      flex_group;           // group for visibility                     (nflex x 1)
   int*      flex_interp;          // interpolation (0: vertex, 1: nodes)      (nflex x 1)
-  int*      flex_bandwidth;       // precomputed solver bandwidth             (nflex x 1)
   int*      flex_cellnum;         // finite cell num per dimension            (nflex x 3)
   int*      flex_nodeadr;         // first node address                       (nflex x 1)
   int*      flex_nodenum;         // number of nodes                          (nflex x 1)

include/mujoco/mjxmacro.h

@@ -455,7 +455,6 @@
     X   ( int,     flex_matid,            nflex,         1                    ) \
     X   ( int,     flex_group,            nflex,         1                    ) \
     X   ( int,     flex_interp,           nflex,         1                    ) \
-    X   ( int,     flex_bandwidth,        nflex,         1                    ) \
     X   ( int,     flex_cellnum,          nflex,         3                    ) \
     X   ( int,     flex_nodeadr,          nflex,         1                    ) \
     X   ( int,     flex_nodenum,          nflex,         1                    ) \