[ad-17] add finite-difference residual HVP fallback

[krystophny]

Closes #18.

Add a finite-difference Hessian-vector product fallback over the public raw residual API:

• State.hessian_vector_product(vector, eps=1e-6, method="fd") returns the centered residual difference at the current state.
• The method validates vector length before perturbing the state.
• Unknown methods raise UnsupportedFeatureError, so the finite-difference path remains explicit and does not pretend to be the future exact Enzyme HVP.
• Tests cover the centered formula, zero vector, state/input preservation, wrong length, and unknown method behavior.

Verification

Test fails on main

On the parent of this PR branch:

$ uv run pytest test-CI/unit-pygvec/test_ad_hvp.py -q --require-pygvec
FFFFF                                                                    [100%]
AttributeError: 'State' object has no attribute 'hessian_vector_product'
5 failed in 0.91s

Test passes after fix

$ uv run pytest test-CI/unit-pygvec/test_ad_hvp.py -q --require-pygvec
.....                                                                    [100%]
5 passed in 9.78s

$ uv run pytest test-CI/unit-pygvec/test_ad_layout.py test-CI/unit-pygvec/test_ad_energy.py test-CI/unit-pygvec/test_ad_raw_force.py test-CI/unit-pygvec/test_ad_context.py test-CI/unit-pygvec/test_ad_public_api.py test-CI/unit-pygvec/test_ad_preconditioner.py test-CI/unit-pygvec/test_ad_hvp.py -q --require-pygvec
...............................sss..s...                                 [100%]
36 passed, 4 skipped in 10.29s

$ git diff --check
# no output

$ fo
fo: no Fortran project detected

[krystophny]

Fixed AD-17 by adding State.hessian_vector_product(..., method="fd") over the public raw residual API. The fallback is explicit, validates vector length before perturbation, preserves state and input vector, returns zero for the zero direction, and rejects non-fd methods so exact Enzyme HVP remains a separate future path.