Migrate KIM special functions to fortnum (1/8)

[krystophny]

Migrate KIM special functions to fortnum (1/8)

Replaces the GSL special-function calls in KIM with the fortnum core. First branch of the cumulative k1..k8 then z1..z3 migration stack; every later branch builds on this one.

Pins the fortnum FetchContent GIT_TAG to main 974dcf1, which carries all kernel fixes plus the two new integrators.

Golden-record CI timeout fix

ql-balance_golden_record ran ensure_golden() at pytest collection time. That clones KAMEL main and does a full from-scratch cmake build of main_ref inside the 600 s timed test, so the comparison only began near 600 s and the test timed out even though the numerics were fine.

The build of main_ref and the reference run now live in a separate CMake setup fixture, ql-balance_golden_setup (timeout 1800 s), wired via FIXTURES_SETUP ql-balance_golden. ql-balance_golden_record requires that fixture and runs only the comparison. test_golden_record.py locates golden.h5 directly and falls back to ensure_golden() only for a bare pytest invocation outside CTest.

Verification

This branch reproduces main bit-for-bit on the golden record:

RESULT: 114/114 pass, 0 fail
worst rel=0.000e+00 abs=0.000e+00

(uv venv: pytest numpy scipy h5py f90nml matplotlib; golden generated from main 428a708; bar rtol=1e-8 atol=1e-15, unchanged.)

Merge order

#140 k1 -> #141 k2 -> #142 k3 -> #143 k4 -> #144 k5 -> #145 k6 -> #146 k7 -> #147 k8 -> #148 z1 -> #149 z2 -> #150 z3.

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