Migrate KiLCA/QL-Balance quadrature to fortnum (3/8)

[krystophny]

Merge order

These fortnum-migration PRs are individually based on main and form one
cumulative stack. Merge them in this order:

#140 -> #141 -> #142 -> #143 -> #144 -> #145 -> #146 -> #147 -> #148 -> #149 -> #150

Each PR is opened against main, so its diff is cumulative versus main and
overlaps its predecessors. Merging in the order above keeps the history linear:
once a predecessor merges into main, the next PR's diff shrinks to just its
own increment.

Scope

Route the C/C++ GSL adaptive-quadrature call sites onto the fortnum C ABI. The Fourier transform (four_transf) and the conductivity drift test (calc_I_array_drift) use fortnum_integrate_qag; the QL-Balance velocity integral (vel_integral) uses fortnum_integrate_qagiu over the doubly infinite interval. GSL workspace alloc/free and gsl_function wrappers are removed; integrand callbacks take fortnum's (x, ctx) ABI. Adds the fortnum C ABI include dir and links fortnum into kilca_lib and ql-balance_lib.

Dependency removed

GSL gsl_integration_qag/qagi in KiLCA and QL-Balance.

Verification

The K3 fortnum quadrature signatures compile and link against libfortnum:

$ g++ -std=c++14 -I.../fortnum/include probe.cpp libfortnum.a -lgfortran -o probe && ./probe
0.5   # int_0^1 x dx via fortnum_integrate_qag / qagiu

Stack tip builds green and ctest passes 18/18 (see K8). Draft: only the stack tip was built end to end.

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Stack reconciliation (update)

The migration stack was rebuilt as a strictly linear cumulative chain on main 428a708:

k1 (+ golden CI fix) -> k2 -> k3 -> k4 -> k5 -> k6 -> k7 -> k8 -> z1 -> z2 -> z3

Every branch now pins fortnum to main 974dcf1 (all kernel fixes plus both new integrators). This branch's standalone golden result above stands. The cumulative tip (#150 z3) does not pass: the KiLCA background equilibrium ODE on #143 k4 cannot reproduce the GSL rk8pd golden with DOP853 at the unchanged 1e-8 bar. See #143 and #150. Tolerances were not weakened and the stored golden was not regenerated.

Merge order: #140 -> #141 -> #142 -> #143 -> #144 -> #145 -> #146 -> #147 -> #148 -> #149 -> #150.

Note: fortnum pin updated to current main (92de6e9) after a fortnum history rewrite; old shas no longer resolve.