Add zero-iteration early exit to the primal solvers, certified by the duality gap.

The primal cost has curvature of at least M in every zone, making it strongly
convex in the M-norm and bounding the suboptimality of any point by the
Fenchel duality gap at its constraint forces:

  cost(qacc) - cost* <= 0.5*grad'*M^-1*grad

Since M's factorization always exists, this certificate is evaluable before
the solver does any work: one triangular solve and one dot product. When the
warmstarted solution is already certified to satisfy the tolerance, CG and
Newton now return with zero iterations; for Newton this skips building and
factorizing the Hessian. If the certificate declines, Newton gets a second
exit after factorization: the Newton decrement, checked before the first
line search.

Because the gap bounds cost suboptimality, stiff constraints can convert it
into force errors of order sqrt(2*gap*stiffness). Newton solutions are
characteristically force-accurate, so Newton zero-iteration exits also
require the gradient criterion, preserving constraint-force accuracy at
rest; CG solutions are characteristically cost-accurate and exit on the gap
alone.

On a settling pile of 50 boxes (300 dofs, ~200 contacts), end-to-end time
per step drops 13% over a settle-then-rest run and 27% in the quiescent
limit, with Newton iterations falling from 0.98 to 0.40 per step.

Tests: WarmstartZeroIterations sweeps solver/cone/jacobian on a settled box,
asserting zero iterations, forward/inverse consistency, and agreement with a
tolerance=0 control solve from the same state. WarmstartZeroIterationsIslands
checks per-island exits with a kicked box next to a settled one.
RefsiteConservesMomentum now requests an exact solve (tolerance=0), since it
asserts momentum conservation tighter than the solver tolerance contract.
PiperOrigin-RevId: 947993735
Change-Id: I2fd855774bff619709b2c386f1ba2714286e0821

doc/computation/index.rst

@@ -1445,6 +1445,14 @@ representations of the constraint Jacobian and related matrices.
    bootstraps the solver when constraints persist across time steps, but avoids carrying over stale forces from
    constraints that have disappeared.
 
+   Because every zone of the piecewise-quadratic cost has curvature of at least :math:`M`, the cost is strongly convex
+   in the :math:`M`-norm, which bounds the suboptimality of any point by the duality gap at its constraint forces:
+   :math:`\text{cost}(a) - \text{cost}^* \le \tfrac{1}{2} g^T M^{-1} g`. Before starting iterations, the CG and Newton
+   solvers evaluate this certificate at the warmstarted point, using the already-computed factorization of
+   :math:`M`. If it is below tolerance, convergence is proven and the solver returns immediately with zero iterations;
+   in the Newton case this skips constructing and factorizing the Hessian. In a quiescent, well-warmstarted scene this
+   eliminates nearly the entire cost of the constraint solver.
+
 .. _soIsland:
 
 Constraint islands