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.

wasm/codegen/generated/bindings.cc

@@ -4609,9 +4609,6 @@ EMSCRIPTEN_BINDINGS(mujoco_bindings) {
     .property("efm_K_rownnz", &MjData::efm_K_rownnz)
     .property("efm_K_val", &MjData::efm_K_val)
     .property("efm_L", &MjData::efm_L)
-    .property("efm_L_colind", &MjData::efm_L_colind)
-    .property("efm_L_rowadr", &MjData::efm_L_rowadr)
-    .property("efm_L_rownnz", &MjData::efm_L_rownnz)
     .property("efm_active", &MjData::efm_active, &MjData::set_efm_active, reference())
     .property("efm_c", &MjData::efm_c)
     .property("efm_dofid", &MjData::efm_dofid)

wasm/codegen/generated/bindings.h

@@ -7314,15 +7314,6 @@ struct MjData {
   emscripten::val efm_dofid() const {
     return emscripten::val(emscripten::typed_memory_view(ptr_->nefmdof, ptr_->efm_dofid));
   }
-  emscripten::val efm_L_rownnz() const {
-    return emscripten::val(emscripten::typed_memory_view(ptr_->nefmdof, ptr_->efm_L_rownnz));
-  }
-  emscripten::val efm_L_rowadr() const {
-    return emscripten::val(emscripten::typed_memory_view(ptr_->nefmdof, ptr_->efm_L_rowadr));
-  }
-  emscripten::val efm_L_colind() const {
-    return emscripten::val(emscripten::typed_memory_view(ptr_->nefmL, ptr_->efm_L_colind));
-  }
   emscripten::val efm_L() const {
     return emscripten::val(emscripten::typed_memory_view(ptr_->nefmL, ptr_->efm_L));
   }