Migrate KIM Fokker-Planck QUADPACK to fortnum (2/8)

[krystophny]

Migrate the KIM Fokker-Planck QUADPACK calls (vendored Netlib dqag/dqags) onto the fortnum adaptive integrator, and pin fortnum to the main commit that makes its QAGS/QAGP/QAGIU path reproduce QUADPACK.

The fortnum adaptive driver previously used a clean-room extrapolation control that drifted on singular and semi-infinite integrands and used 5-11x more panels than QUADPACK at the same tolerance, which is what timed out this golden. fortnum main 974dcf1 reimplements the extrapolation path to follow QUADPACK dqagse/dqagpe (faithful dqelg Wynn-epsilon table, incremental dqpsrt ordering, depth-keyed level/levmax extrapolation gate, break-adjacent ndin error inflation). int_0^1 ln x and int_0^1 x^-1/2 now reach machine precision in 6 subintervals (231 GK21 evals), bit-matching Netlib dqagse.

Verification

Built against fortnum main 974dcf1 and ran the golden under a uv venv (pytest numpy scipy h5py f90nml matplotlib), Python 3.13.

Before (CI, this branch on the old fortnum): ql-balance_golden_record exceeded the 600 s timeout (the slow over-subdividing QAGS/QAGIU path).

After:

2/2 Test #19: ql-balance_golden_record .........   Passed  241.06 sec
100% tests passed, 0 tests failed out of 2

Per-quantity pytest run (golden bar rtol=1e-8, atol=1e-15, unchanged):

============================= 115 passed in 49.07s =============================

All LinearProfiles/KinProfiles quantities including Br_Re/Br_Im/Br_abs, dqle*/dqli*, T_EM_phi_e/i pass; no quantity exceeds rtol=1e-8.

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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.