Replace the flex metric factorization with a block preconditioner

Every step, the flex block of the implicit effective metric M + K was
factorized by sparse Cholesky, because K depends on the configuration. On
model/flex/bag.xml, added here, that is roughly half the step, against a
comparable share for the constraint solve it exists to accelerate.

Keep only the metric's per-vertex 3x3 diagonal blocks, prefactored. Neither
consumer needs the exact inverse: the CG constraint solver only wants a
preconditioner, and qacc_smooth can come from an iterative solve using those
blocks. They are O(n) to build and to apply, but weaker, so CG runs about twice
the iterations and qacc_smooth becomes an iteration rather than a direct solve.
Net, the bag model steps roughly twice as fast.

The preconditioner, by metric state. Inactive, meaning no flex elasticity or an
explicit integrator: M^-1, unchanged. Bending only (nefmK == 0): M^-1 plus the
exact constant bending factor from mj_setConst on the dofs it covers,
unchanged; that factor is built at model compile time and costs nothing per
step. Per-step stiffness: M^-1 plus the 3x3 blocks, where before it was a
per-step sparse Cholesky, or, when M couples across the flex block, an inner
PCG of up to 50 iterations run once per outer CG iteration.

Only models carrying per-step stretch stiffness change in wall-clock. Both
ponchos hold their timing and take slightly fewer CG iterations than before,
because the preconditioner is now symmetric: it applies M^-1 and the covered
blocks to disjoint sets of dofs, where previously the two overlapped and the
operator was not symmetric, which PCG requires.

mjd_effSolve is the accurate solve of (M + K)x = b; what used to carry that
name only preconditions and is now mjd_effPrec. Its CG guarded the division by
pAp with mjMINVAL, an absolute floor on a quantity that scales with the square
of the right-hand side, so a small b aborted the solve while the curvature was
healthy: four flex models were quietly left short of tolerance. For an SPD
metric the guard is positivity, and with that the same solves converge. The qacc_smooth call site in
mj_fwdAcceleration is textually unchanged but now reaches the iterative solve,
which converges on opt.tolerance rather than a hardcoded threshold, floored in
mjUSESINGLE builds where the squared target is unreachable in float. Reaching
the iteration cap names the ill-conditioned flex stiffness and then reports it
through mjWARN_INERTIA, rather than returning an under-converged result.
Covered dofs are located by walking the covered rows of the stiffness matrix,
as they need not be 3-aligned from dof 0: any joint declared before a flexcomp
shifts them.

mjData.efm_L_rownnz, efm_L_rowadr and efm_L_colind described the sparsity of
the deleted factorization and are removed: left NULL with nonzero mjxmacro
extents they made the Python bindings hand back uninitialized arrays.
efm_active loses the value 2 for the same reason, nothing selects a solve path
on preconditioner exactness any more. Both are recorded under breaking changes.

model/flex/bag.xml is added because no shipped model carried per-step stretch
stiffness. The ponchos are bending-only and trampoline.xml uses an explicit
integrator, so the metric never activates there. It is excluded from
WriteReadCompareTest: stretch stiffness amplifies rest geometry that XML rounds
on save.

doc/changelog.rst

@@ -2,6 +2,34 @@
 Changelog
 =========
 
+Upcoming version (not yet released)
+-----------------------------------
+
+Engine
+^^^^^^
+
+- Replaced the per-step sparse Cholesky factorization of the flex block of the implicit effective metric M + K with
+  its prefactored per-vertex 3x3 diagonal blocks. The blocks precondition the CG constraint solver and drive an
+  iterative solve for ``qacc_smooth``, which now converges on :ref:`tolerance<option-tolerance>` rather than a fixed
+  threshold. Flexes with :ref:`elastic2d<flex-elasticity-elastic2d>` stretch stiffness step roughly twice as fast;
+  bending-only flexes keep the exact constant factor and are unchanged.
+
+.. admonition:: Breaking API changes
+   :class: attention
+
+   - Removed ``mjData.efm_L_rownnz``, ``mjData.efm_L_rowadr`` and ``mjData.efm_L_colind``. They described the sparsity
+     of the effective-metric Cholesky factor, which no longer exists; ``mjData.efm_L`` now holds dense 3x3 blocks,
+     9 numbers per covered vertex. ``mjData.efm_active`` no longer takes the value 2: nothing selects a solve path on
+     preconditioner exactness, so it is now a plain 0/1 flag.
+
+Models
+^^^^^^
+
+- Added `bag <https://github.com/google-deepmind/mujoco/blob/main/model/flex/bag.xml>`__ example model: a cloth bag,
+  held open by pinning the ring of vertices around its mouth, catching the standard humanoid dropped in from above.
+  Unlike the poncho models, which are bending-only, this model exercises the 2D
+  :ref:`stretch<flex-elasticity-elastic2d>` elasticity of a flex.
+
 Version 3.11.0 (July 27, 2026)
 ------------------------------