Add non-energy boundary adjoint example

[krystophny]

Chains the non-energy objective gradient through the equilibrium boundary adjoint.

• simsopt_gvec_objective.py composes GvecEnergyProblem's cached interior re-solve with State.objective_state_gradient and adjoint_boundary_gradient, giving an analytic boundary gradient for bsq_variance with no finite-difference re-solves.
• SIMSOPT Optimizable wrapper GvecObjective with J()/dJ() and an L-BFGS-B optimize_boundary loop.
• Sampled validation against centered boundary perturbations with full interior re-solves.

Closes #27.

Verification

Test fails before

$ pytest test-CI/unit-pygvec/test_ad_simsopt.py -q --require-pygvec
5 passed in 1.18s   # no non-energy gradient tests, simsopt_gvec_objective.py missing

Test passes after

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

$ pytest test-CI/unit-pygvec/test_ad_simsopt.py -q --require-pygvec
7 passed in 1.63s

test_non_energy_boundary_gradient_sampled re-solves the interior for every boundary DOF and requires cosine similarity above 0.99 and rtol=5e-3 between the adjoint gradient and the finite-difference truth.

Enzyme-off isolated build:

$ pytest test-CI/unit-pygvec/test_ad_simsopt.py -q --require-pygvec
5 passed, 2 skipped in 2.00s