Integrate the equilibrium ODE with fortnum DOP853 instead of ddeabm (Z2)

[krystophny]

Integrate the equilibrium ODE with fortnum ddeabm (Z2)

KIM/src/background_equilibrium/calculate_equil.f90 now integrates the scalar force-balance ODE du/dr with the fortnum variable-order Adams integrator (fortnum_ode_ddeabm), a clean-room equivalent of SLATEC ddeabm. This replaces the earlier DOP853 substitution and restores the original SLATEC continuation semantics.

One ddeabm_init seeds the state at r_grid(1). Each outer grid value u(i) continues the same re-entrant state to r_grid(i) (the SLATEC INFO(1)=1 restart), with the step barred from overshooting r_grid(end) (the RWORK(1) / INFO(4)=1 tstop bound). idid < 1 maps to the existing log_warning path via ode_status%code /= FORTNUM_OK.

The original scalar RHS dudr(r, u, du) is wrapped as the array-valued ode_rhs_t callback dudr(r, u, du, ctx) with neq = 1; the q-profile and pressure data ride on host association, so ctx is unused.

Cumulative on #148 z1; fortnum pinned to main 974dcf1.

Verification

This rewire compiles and runs on the cumulative tip. The KIM equilibrium quantities it feeds are not isolated in the golden comparison set; the tip golden failures all trace to the KiLCA background ODE on #143 k4 (see that PR), not to this change. No new failure category appears at this branch.

Tolerances unchanged (rtol=1e-12, atol=1e-12 for this ODE; golden bar rtol=1e-8 atol=1e-15).

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