[ad-21] add forward boundary sensitivity helpers

[krystophny]

Adds forward fixed-boundary sensitivity helpers for the public pyGVEC AD surface.

• Selects fixed-boundary design DOFs and excludes LA coefficients.
• Solves the interior response system with State.raw_residual_jvp as the linear operator (exact when the build provides it, FD otherwise); the residual-JVP naming keeps the raw-residual sign explicit.
• Applies the forward chain rule for scalar objective gradients.
• Checks sampled sensitivities against finite-difference boundary perturbations with an interior re-solve that preserves the current interior raw residual.

Closes #22.

Verification

Test fails before

$ bash -lc 'test -f test-CI/unit-pygvec/test_ad_boundary.py || { echo "missing boundary sensitivity test before AD-21"; exit 2; }'
missing boundary sensitivity test before AD-21

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.

[krystophny]

Implemented AD-21 here.

• Added python/examples/gvec_adjoint.py for fixed-boundary forward sensitivity solves with the public HVP operator.
• Added test-CI/unit-pygvec/test_ad_boundary.py covering design DOF selection, interior response solves, scalar forward gradients, and sampled finite-difference re-solves.

Verification: focused boundary pytest passed (4 passed), adjacent AD API/HVP/transpose/boundary pytest passed (19 passed).