Add production Enzyme gradient

[krystophny]

Summary

• compile the existing fixed-map flat energy kernel and its Enzyme gradient into libGVEC
• reuse the generated Fortran modules instead of maintaining a second kernel implementation
• replace the standalone AD fixture with a test of the production library symbols

Verification

Failing before on fd3b3d0:

$ nm -g --defined-only <parent-build>/lib/libGVEC.a | rg ' [Tt] gvec_mhd3d_flat_energy_gradient$'
<no output; exit 1>

Passing after:

$ fo
Static: OK (96 modules, 96 changed, 96 affected)
Build: OK
Tests: skipped, no affected tests
Lint: OK
All stages passed (.4s)

$ ctest --test-dir <rz-build> --output-on-failure -R '^gvec_enzyme_'
100% tests passed, 0 tests failed out of 5

$ ctest --test-dir <cyl-build> --output-on-failure -R '^gvec_enzyme_'
100% tests passed, 0 tests failed out of 5

Fresh RZ and cylindrical builds completed 152/152 targets. Each archive contains one production energy definition and one production gradient definition, with no unresolved __enzyme_* symbols.