Validate Enzyme gradient against raw force

[krystophny]

Summary

• Extend gvec_enzyme_ad_fixture_gradient to fetch EvalForceRawProjected through pyGVEC.
• Compare Enzyme dW/dq to -raw_force on all free DOFs reported by the existing interior-DOF API.
• Keep boundary-constrained DOFs excluded from the raw-force sign check.

Verification

Test fails before this PR

rg 'get_raw_projected_force|rawForce|raw_force' test-CI/enzyme/gvec_enzyme_ad_fixture_gradient.F90
# no matches, exit code 1

Test passes after fix

cmake --build /tmp/gvec-enzyme-ad10f --target gvec_enzyme_ad_fixture_gradient -j$(nproc)
ctest --test-dir /tmp/gvec-enzyme-ad10f -R 'gvec_enzyme_ad_fixture_gradient' --output-on-failure

100% tests passed, 0 tests failed out of 1

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

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

fo
fo: no Fortran project detected