Export AD field diagnostic arrays

[krystophny]

Exports Enzyme-ready field diagnostic arrays from the fixed-map flat context.

• Adds EvalFieldDiagnosticsFlatKernel beside the energy kernel: per-quadrature-point sqrtG, |B|, B^theta, B^zeta from the same flat context, allocation-free with caller-owned buffers.
• Fixed-map dispatch gvec_mhd3d_flat_field_diagnostics in the AD adapter; cached-context wrapper eval_field_diagnostics in MODgvec_py_state_ad.
• State.ad_diagnostics(names) returns a dict of flat arrays; unknown names raise the new MissingQuantityError; non-Enzyme builds raise UnsupportedFeatureError.
• Behavioral check: the diagnostics reconstruct the total MHD energy to rtol=1e-10 against State.energy.

Closes #25.

Verification

Test fails before

$ bash -lc 'test -f test-CI/unit-pygvec/test_ad_objectives.py || { echo "missing diagnostics test before AD-24"; exit 2; }'
missing diagnostics test before AD-24

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
4 passed in 1.41s

Enzyme-off isolated build:

$ pytest test-CI/unit-pygvec/test_ad_objectives.py -q --require-pygvec
2 passed, 2 skipped in 0.87s

Fresh fixed-RZ Enzyme ctest: 6/6.