[ad-16] expose GVEC preconditioner operator

[krystophny]

Closes #17.

Expose the current-state GVEC preconditioner as a reusable flat-vector operator:

• State.apply_preconditioner(vector) validates the shared flat layout and returns a new transformed vector.
• The pyGVEC wrapper assembles the MHD3D preconditioner from the current state without changing q.
• The Fortran operator applies boundary RHS transforms and the factorized preconditioner matrices to the supplied vector, not to the solver force.
• Builds with PrecondType <= 0 raise a typed UnsupportedFeatureError instead of silently returning a non-preconditioned solver transform.

Verification

Test fails on main

On the parent of this PR branch:

$ uv run pytest test-CI/unit-pygvec/test_ad_preconditioner.py -q --require-pygvec
FFFFFFF                                                                  [100%]
AttributeError: 'State' object has no attribute 'apply_preconditioner'
7 failed in 0.90s

Test passes after fix

$ uv pip install -e .
Built gvec
Installed gvec

$ uv run pytest test-CI/unit-pygvec/test_ad_preconditioner.py -q --require-pygvec
........                                                                 [100%]
8 passed in 12.82s

$ 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 -q --require-pygvec
..........................sss..s...                                      [100%]
31 passed, 4 skipped in 13.26s

$ git diff --check
# no output

$ fo
fo: no Fortran project detected

[krystophny]

Fixed AD-16 by adding State.apply_preconditioner(vector), backed by the existing MHD3D preconditioner matrices rather than solver-force evaluation. The operator validates flat-vector layout, preserves the loaded state and input vector, is deterministic and linear on the fixture, and raises UnsupportedFeatureError when PrecondType disables the preconditioner.