Solve interior with exact-JVP Newton-Krylov

[krystophny]

Replaces the finite-difference-Jacobian hybr interior re-solve in the SIMSOPT wrappers with a damped Newton-Krylov solver.

• Matvecs come from State.raw_residual_jvp (exact on Enzyme builds, centered FD on default builds), preconditioned by State.apply_preconditioner; no dense O(n^2) FD Jacobian remains anywhere on the solve path.
• The equilibrium map now drives the interior raw residual to force balance instead of preserving the fixture's residual target, matching VMEC++'s solve_interior semantics: the energy becomes stationary with respect to interior solve error, which makes finite-difference cross-checks of the boundary gradient well conditioned.
• Backtracking line search with stall-accept (VMEC++ pattern) plus an acceptance ceiling: a stalled solve above accept_tol * |F0| raises and triggers the penalty contract.
• Counters residual_evaluations and jvp_evaluations make solver cost measurable.

Default builds keep working: the same Newton-Krylov runs with FD matvecs and the public API is unchanged.

Verification

Before

GvecEnergyProblem.solve_equilibrium called scipy.optimize.root(method="hybr"), whose MINPACK Jacobian costs O(n^2) FD residual evaluations per solve (~20 s per solve on the converted solovev case) and fails on stiff cases.

After

Enzyme-on isolated build (fixed RZ, LLVM/Flang 22.1.6):

$ pytest test-CI/unit-pygvec/test_ad_simsopt.py test-CI/unit-pygvec/test_ad_objectives.py -q --require-pygvec
16 passed in 2.66s

Enzyme-off isolated build:

$ pytest test-CI/unit-pygvec/test_ad_simsopt.py test-CI/unit-pygvec/test_ad_hvp.py test-CI/unit-pygvec/test_ad_boundary.py test-CI/unit-pygvec/test_ad_external_optimizers.py -q --require-pygvec
27 passed, 2 skipped in 25.92s

test_newton_interior_solve_reaches_force_balance drives the fixture (initial interior residual norm 3.78) below 1e-3 relative with fewer than 20 residual evaluations per Newton iteration. test_simsopt_gradient_matches_fd_resolve validates the adjoint boundary gradient against centered FD with full re-solves at rtol=1e-5.