Migrate KiLCA roots, sort, multiroot to fortnum (5/8)

[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 remaining GSL solver and utility call sites onto the fortnum C ABI. The resonant-surface search (calc_mode) uses fortnum_root_brent; the eigenmode determinant zero search (calc_eigmode) uses fortnum_multiroot_hybrid with its internal central-difference Jacobian in place of the gsl_multiroot_fdfsolver hybridsj solver plus the hand-rolled eval_jac; the grid index sorts (calc_cond, sysmat_profs, adaptive_grid) go through a new sort_index_doubles helper wrapping fortnum_argsort. The gsl_vector residual and the unused GSL circle-integration helpers are removed.

Dependency removed

GSL roots (gsl_root_fsolver brent), multiroot (gsl_multiroot_fdfsolver hybridsj), gsl_deriv_central, and gsl_heapsort in KiLCA.

Verification

The fortnum root/multiroot/deriv/argsort signatures compile, link, and compute correctly:

$ g++ -std=c++14 -I.../fortnum/include probe.cpp libfortnum.a -lgfortran -o probe && ./probe
brent=1.41421 root2(1,2) deriv=2 perm=1,2,0

test_root_finding, test_ampere_matrices pass in the stack-tip ctest run (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.