Replace bundled AMOS complex Bessel with fortnum (6/8)

[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

The headline removal: replace the bundled AMOS complex-argument Bessel sources with the fortnum complex-Bessel core. A new common/math/fortnum_amos_compat module provides zbesj/zbesi/zbesk on the historical AMOS (ZR, ZI, FNU, KODE, N, CYR, CYI, NZ, IERR) ABI, so every caller compiles and links unchanged: the KiLCA wrappers (interface/bessel.f90, math/bessel/bessel.f90), the conductivity D-array kernels, the vacuum stitching (hmedium), the Maxwell start values, and the KIM dispersion solvers (fun_input, wkb_dispersion). The zbes* family and its AMOS helpers (and dgamln, used only by them) are deleted from common/math/slatec; the machine-constant and error routines stay for QUADPACK and ddeabm. The dead, never-compiled KiLCA/math/bessel/amos and /644 trees are deleted.

Dependency removed

Bundled AMOS zbesj/zbesi/zbesk (common/math/slatec z*.f) and the SLATEC zbesi used by KIM.

Verification

The fortnum-backed shim computes the AMOS sequences correctly through the original ABI:

$ gfortran -I.../fortnum/include fortnum_amos_compat.f90 probe.f90 libfortnum.a -o probe && ./probe
 zbesi I0..I2 re=   1.1798566  0.5124138  0.0885864  ierr= 0
 zbesk K0..K2 re=   0.3078189  0.3763245  0.5884542  ierr= 0
 zbesj J1(2)=  0.5767248  0.0  ierr= 0   (bessel_j1(2)=0.576725)

In the stack-tip build, nm over every shipped binary shows 0 defined AMOS implementation symbols (zbknu zbinu zseri zmlri zasyi zacai zacon zunhj zunik zuoik zwrsk zbuni zbunk zairy zbesh zbesy zbiry dgamln zabs all 0), while zbesi_/zbesj_/zbesk_ resolve to the fortnum-backed shim and pull fortnum_bessel_i_complex_array / __fortnum_special_complex_bessel_MOD_*. test_ampere_matrices and the integration tests pass (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.