Add opt-in Enzyme CI checks

[krystophny]

Adds an opt-in CI path for the Enzyme toolchain.

• Labels every gvec_enzyme_* CTest with enzyme so ctest -L enzyme selects the smoke, fixed-RZ gradient, and HVP validation set.
• New .github/workflows/enzyme.yml: manual dispatch plus pull requests touching AD paths; installs LLVM 21 from apt.llvm.org, builds and caches Enzyme v0.0.264, configures the fixed-RZ Enzyme build, runs ctest -L enzyme, and uploads the CTest log on failure.
• Normal builds and CI on machines without Enzyme are unaffected; the workflow only triggers on dispatch or AD-path changes.
• Documents the CI toolchain in docs/user/differentiable-gvec.md.

Closes #29.

Verification

Before

$ ctest --test-dir <build> -L enzyme -N
Total Tests: 0

After

$ ctest --test-dir <build-enzyme-rz> -L enzyme -N
  Test #1: gvec_enzyme_smoke
  Test #2: gvec_enzyme_array_gradient
  Test #3: gvec_enzyme_rz_flat_gradient
  Test #4: gvec_enzyme_energy_gradient_mpi
  Test #5: gvec_enzyme_gvec_module_smoke
  Test #6: gvec_enzyme_hvp
Total Tests: 6

The workflow itself runs on GitHub Actions when dispatched; the first dispatch after merge validates REQ-AD28-001 on a hosted worker.