Add Enzyme objective state gradient

[krystophny]

Makes one non-energy objective differentiable with respect to the flat state.

• Adds EvalBsqVarianceFlatKernel: volume-weighted variance of |B|^2, computed single-pass from the same flat context and with the same argument signature as the energy kernel.
• Reverse-mode gradient gvec_mhd3d_flat_bsq_variance_gradient through the existing __enzyme_autodiff interface; cached-context wrappers eval_bsq_variance and eval_bsq_variance_gradient.
• State.objective_value("bsq_variance", q) and State.objective_state_gradient("bsq_variance", q, method="exact"|"fd"); the FD path differentiates the fixed-state objective, never an equilibrium re-solve.
• Unknown objective names raise MissingQuantityError; non-Enzyme builds raise UnsupportedFeatureError.

Closes #26.

Verification

Test fails before

$ pytest test-CI/unit-pygvec/test_ad_objectives.py -q --require-pygvec
4 passed in 1.41s   # no objective tests, State.objective_value missing

Test passes after

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

$ pytest test-CI/unit-pygvec/test_ad_objectives.py -q --require-pygvec
7 passed in 1.00s

test_objective_state_gradient_matches_fd checks the exact reverse-mode gradient against per-component centered differences at rtol=1e-5. test_objective_value_bsq_variance reconstructs the objective from the exported diagnostics at rtol=1e-10.

Enzyme-off isolated build:

$ pytest test-CI/unit-pygvec/test_ad_objectives.py -q --require-pygvec
4 passed, 3 skipped in 0.93s

Fresh fixed-RZ Enzyme build has exactly one gvec_mhd3d_flat_bsq_variance and one gvec_mhd3d_flat_bsq_variance_gradient symbol; ctest 6/6.