Replace ZEAL with fortnum complex_region_roots in WKB dispersion (Z1)

[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.

First of three stacked PRs (Z1-Z3) that finish removing the avoidable upstream numerical dependencies from KIM onto fortnum, completing the GSL/AMOS/SLATEC/QUADPACK removal. Base of this PR is the fortnum k8-plumbing tip (PR #147).

What changed

run_ZEAL_dispersion drove the bundled ZEAL package (zeal(), set_zeal_search_region, the Zeal_Module/Zeal_Input_Module/Function_Input_Module triple, VALREG, and the netlib dqagx contour quadrature) to find the complex zeros of the kinetic dispersion relation over a rectangular search box. It now drives fortnum complex_region_roots over the same box (center +/- halfwidth maps to the ll/ur corners). The per-branch focused search, the broad search, multiplicities, and the WKB_dispersion_solver='ZEAL' config option are preserved; only the dependency internals change.

The KIM dispersion functions move onto the fortnum complex_root_fn_t ABI via the dispersion_region_fn adapter. Function_Input_Module drops its Precision_Module dependency (uses KIM_kinds_m) and the ZEAL-only VALREG. zeal_input.f90, test_zeal.f90, FetchZeal.cmake, the external_zeal source set, and the now-unused dqagx source are removed.

fortnum is pinned to 38a5110, which provides complex_region_roots and the LAPACK-backed eigensolve, guarded by if(NOT TARGET fortnum).

Verification

Full CMake/Ninja build at the stack tip (fortnum 38a5110), then ctest. The new test_region_roots_vs_muller cross-checks that complex_region_roots and the in-tree Muller solver return the same zeros on one analytic test function to within 1e-6.

Before (on the base branch the ZEAL package is fetched and compiled; no region/Muller equivalence test exists).

After:

$ ./build/tests/test_region_roots_vs_muller.x
 Region/Muller equivalence OK:            3  shared zeros within   9.9999999999999995E-007
   zero            1  =  (-2.0000000000000000,1.0000000000000000)  mult 1
   zero            2  =  (0.25000000000000000,-1.7500000000000000) mult 1
   zero            3  =  (1.5000000000000000,0.50000000000000000)  mult 1

$ ctest
100% tests passed, 0 tests failed out of 19

$ nm build/install/bin/KIM.x | grep -ciE 'zeal_module|zeal_input_module|valreg|set_zeal_search'
0

---

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.