Test Enzyme gradient on AD fixture

[krystophny]

Summary

• Add gvec_enzyme_ad_fixture_gradient, an Enzyme test that initializes the low-resolution AD fixture outside the differentiated region.
• Pass the exported fixed-RZ flat-kernel context as inactive Enzyme inputs and differentiate only q.
• Compare the Enzyme dW/dq against centered finite differences for all fixture q entries.

Verification

Test fails before this PR

ENZYME_PLUGIN_DIR=<LLVM Enzyme plugin dir> cmake -S . -B /tmp/gvec-enzyme-ad10f-before -G Ninja -DCMAKE_Fortran_COMPILER=flang-new -DCMAKE_BUILD_TYPE=Release -DGVEC_FIX_HMAP=RZ -DUSE_OPENMP=OFF -DLINK_GVEC_TO_NETCDF=OFF -DUSE_ENZYME=ON
cmake --build /tmp/gvec-enzyme-ad10f-before --target gvec_enzyme_ad_fixture_gradient -j$(nproc)

ninja: error: unknown target 'gvec_enzyme_ad_fixture_gradient'

Test passes after fix

ENZYME_PLUGIN_DIR=<LLVM Enzyme plugin dir> cmake -S . -B /tmp/gvec-enzyme-ad10f -G Ninja -DCMAKE_Fortran_COMPILER=flang-new -DCMAKE_BUILD_TYPE=Release -DGVEC_FIX_HMAP=RZ -DUSE_OPENMP=OFF -DLINK_GVEC_TO_NETCDF=OFF -DUSE_ENZYME=ON
cmake --build /tmp/gvec-enzyme-ad10f --target gvec_enzyme_smoke gvec_enzyme_array_gradient gvec_enzyme_rz_flat_gradient gvec_enzyme_gvec_module_smoke gvec_enzyme_ad_fixture_gradient -j$(nproc)
ctest --test-dir /tmp/gvec-enzyme-ad10f -R 'gvec_enzyme_(smoke|array_gradient|rz_flat_gradient|gvec_module_smoke|ad_fixture_gradient)' --output-on-failure

100% tests passed, 0 tests failed out of 5

ENZYME_PLUGIN_DIR=<LLVM Enzyme plugin dir> cmake -S . -B /tmp/gvec-enzyme-ad10f-nogvec -G Ninja -DCMAKE_Fortran_COMPILER=flang-new -DCMAKE_BUILD_TYPE=Release -DCOMPILE_GVEC=OFF -DCOMPILE_SRC_PYGVEC=OFF -DLINK_GVEC_TO_NETCDF=OFF -DUSE_OPENMP=OFF -DUSE_ENZYME=ON
cmake --build /tmp/gvec-enzyme-ad10f-nogvec --target gvec_enzyme_smoke gvec_enzyme_array_gradient gvec_enzyme_rz_flat_gradient gvec_enzyme_gvec_module_smoke -j$(nproc)
ctest --test-dir /tmp/gvec-enzyme-ad10f-nogvec -R 'gvec_enzyme_(smoke|array_gradient|rz_flat_gradient|gvec_module_smoke)' --output-on-failure

100% tests passed, 0 tests failed out of 4

cmake -S . -B /tmp/gvec-build-ad10f -G Ninja -DCMAKE_BUILD_TYPE=Release -DGVEC_FIX_HMAP=RZ -DUSE_OPENMP=OFF -DLINK_GVEC_TO_NETCDF=OFF
cmake --build /tmp/gvec-build-ad10f -j$(nproc)

SUCCESS: GVEC BUILD COMPLETE!
SUCCESS: GVEC_POST BUILD COMPLETE!

git diff --check -- CMakeLists.txt test-CI/enzyme/gvec_enzyme_ad_fixture_gradient.F90
# no output

fo
fo: no Fortran project detected