Add reverse boundary adjoint

[krystophny]

Adds the one-solve reverse boundary adjoint for scalar objectives.

• Solves the interior transpose system through the tested symmetric raw-Hessian contract.
• Returns the direct boundary derivative minus the interior coupling term.
• Matches forward sensitivities on every boundary degree of freedom.
• Matches MHD-energy finite differences that re-solve the interior for sampled boundary perturbations.
• Uses State.raw_residual_jvp for all matvecs: exact Enzyme derivatives when the build provides them, centered FD otherwise. The adjoint solve uses rtol=1e-7.

Closes #23.

Verification

Test fails before

$ pytest test-CI/unit-pygvec/test_ad_boundary.py -q --require-pygvec -p no:cacheprovider
2 failed, 4 passed in 2.04s

Both failures report that adjoint_boundary_gradient is missing.

Test passes after

Enzyme-off isolated build (FD path):

$ pytest test-CI/unit-pygvec/test_ad_boundary.py -q --require-pygvec
6 passed in 3.51s

Enzyme-on isolated build (exact path, fixed RZ, LLVM/Flang 22.1.6):

$ pytest test-CI/unit-pygvec/test_ad_boundary.py -q --require-pygvec
6 passed in 1.03s

Full stack AD suite at this tip: 31 passed on both builds.