[ad-18] add production Enzyme energy HVP

[krystophny]

Summary

• Generate a production forward-over-reverse energy HVP symbol (gvec_mhd3d_flat_energy_hvp) from the fixed-map kernel with __enzyme_fwddiff over the reverse-mode gradient.
• Cache a gradient work buffer in MODgvec_py_state_ad and expose eval_energy_hvp_enzyme with size and context status checks.
• Add State.energy_hessian_vector_product(q, v, method=...) and State.raw_residual_jvp(q, v, method=...) with the sign contract raw_residual_jvp == -energy_hvp; State.hessian_vector_product(v) stays as the current-state residual-JVP compatibility API.
• Rename the production gradient test executable to gvec_enzyme_production_operators, register it as CTest gvec_enzyme_hvp, and extend it with an HVP check against a centered finite difference of the raw projected force; no duplicate executable run.
• Enzyme-off builds keep the API and raise a typed UnsupportedFeatureError for exact requests.

Closes #19.

Verification

Toolchain: LLVM/Flang 22.1.6, Enzyme LLVMEnzyme-22.so built from commit c9650834.

Before (b678d3a)

No production HVP symbol and no production HVP test:

$ nm lib/libGVEC.a | grep -E " T gvec_mhd3d_flat_energy"
00000000000020c0 T gvec_mhd3d_flat_energy
0000000000002220 T gvec_mhd3d_flat_energy_gradient
$ ctest -N -R production
  Test #6: gvec_enzyme_production_gradient

After (8825058)

Fresh fixed-RZ and fixed-cyl Flang/Enzyme builds (USE_ENZYME=ON, static lib, no netCDF):

RZ:  100% tests passed, 0 tests failed out of 6  (ctest -R '^gvec_enzyme_')
cyl: 100% tests passed, 0 tests failed out of 6
OpenMP RZ: 100% tests passed, 0 tests failed out of 7
           incl. gvec_enzyme_hvp (OMP_NUM_THREADS=1) and gvec_enzyme_hvp_omp4 (OMP_NUM_THREADS=4)

Symbol audit, both hmaps: exactly one definition each and no unresolved Enzyme markers.

$ nm lib/libGVEC.a | grep -E " T gvec_mhd3d_flat_energy"
1 gvec_mhd3d_flat_energy
1 gvec_mhd3d_flat_energy_gradient
1 gvec_mhd3d_flat_energy_hvp
$ nm lib/libGVEC.a | grep -c " U __enzyme"
0

Isolated Python builds:

Enzyme-off: test_ad_public_api.py + test_ad_hvp.py -> 15 passed
Enzyme-on (fixed RZ, Flang-built f90wrap): 15 passed

Exact HVP against centered FD on ad_elliptok_lowres (n_dof=20, seed 42):

HVP exact vs FD: rel diff 4.760e-09, cosine 1.000000000000
raw_residual_jvp == -energy_hvp: True (bitwise)
energy gradient norm 26.115398458257

fo full pipeline passed.

[krystophny]

Implemented the AD-18 validation slice here: the low-resolution AD fixture now computes an Enzyme forward-over-reverse HVP and checks the residual sign contract on interior DOFs against centered raw-force differences. Verification: Enzyme CTest surface is 9/9 passing, including gvec_enzyme_ad_fixture_hvp and _omp4; uv run pytest test-CI/unit-pygvec/test_ad_hvp.py -q --require-pygvec is 5/5 passing; git diff --check is clean; fo reports no Fortran project detected.