pybind: expose the unpreconditioned internal-basis gradient

[krystophny]

Stacked PR — part 14/19 of the differentiable-VMEC++ series. based directly on main.
Diff is cumulative (includes ancestor commits) because the branches are stacked on the fork; review the net change described below.

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What

Add a precondition flag to VmecModel.evaluate (default true, so existing
behaviour is unchanged). With precondition=false the forward model returns at
the existing INVARIANT_RESIDUALS checkpoint, so get_forces() returns the raw,
unpreconditioned force.

Why

This is the first step toward driving VMEC++ with gradient- and Hessian-based
optimizers (inside the solver and as a differentiable component of an external
stellarator optimizer). Such optimizers need a consistent (state, gradient) pair.

VMEC minimizes an augmented functional: the MHD energy plus the Hirshman
spectral-condensation constraint and the lambda (magnetic-coordinate) constraint.
Its "force" is the gradient of that augmented functional with respect to the
decomposed, internal-basis state, where the scalxc scaling makes the state and
force conjugate. The raw force at the INVARIANT_RESIDUALS checkpoint is exactly
that gradient.

The native solver instead exposes the preconditioned search direction
(precondition=true), which is a different vector: the preconditioner is a
non-trivial metric, not a scalar. An external optimizer minimizing in VMEC's
basis needs the raw gradient, plus VMEC's preconditioner applied separately as
the metric (a later patch).

No core solver code changes; only the pybind layer selects the checkpoint.

Verification

evaluate(1, 2) is the documented cold-start case (forces initialised to 1.0);
the raw-gradient path uses iter1 >= 2, where the force is well defined.

Empirically, on examples/data/solovev.json (ns=11), the raw force is the
equilibrium residual: it vanishes once the native solver converges, and it is
not the preconditioned direction.

initial guess : ||raw force|| = 1.718e-01
converged     : ||raw force|| = 5.093e-09     (3.4e7 reduction)
cos(raw, preconditioned) = -0.10             (different direction, not scale)

New tests (tests/test_internal_gradient.py):

test_raw_force_differs_from_preconditioned PASSED
test_raw_force_vanishes_at_equilibrium     PASSED
test_cold_start_is_excluded                PASSED
3 passed

Tracking: #590