Recompute z0 after interpolation and add collision-frequency scale

[marjohma]

Summary

Extracts the two salvageable pieces of the retired wp5 flux-pumping stack onto the forced-periodicity lineage. The rest of that stack (#177–#181, #184) is closed as superseded.

Krook argument z0 (was #183)

interpolate_plasma_backs interpolated the derived z0 alongside the primitives. z0 carries a 1/|k_parallel| singularity at the resonance, so a four-point Lagrange stencil spanning k_parallel = 0 returns values that violate its own defining formula. The fix interpolates primitives only and recomputes

z0 = -(omega_E - omega - i nu) / (|k_parallel| sqrt(2) v_T)

on the target grid — the same invariant the periodized-background path already follows.

Scope: z0 is consumed only by Krook_kernel_plasma_prefacs.f90. The Fokker-Planck path recomputes x1/x2/A1/A2/I^{kl} after interpolation and never reads z0, so FP results are bit-unchanged. This repairs the Krook hat path, whose nml still marks it (unstable) — plausibly part of why.

Collision-frequency scale (was #182)

New positive &KIM_CONFIG key collision_frequency_scale multiplying the calculated electron and ion collision frequencies before z0 and the FP susceptibility inputs are assembled. Default 1.0 leaves every collision formula, profile, unit, and output unchanged. Non-positive values are rejected at startup; the value is displayed when it differs from 1.0 and stored in HDF5 provenance. Same shape as the existing ion_flr_scale_factor knob. No benchmark-acceptance claim — it is a sensitivity parameter.

Tests

test_collision_scale_z0 (new) asserts the z0 invariant on a stencil straddling k_parallel = 0 and away from it, and that the scale reaches both nu and z0 (including the imaginary part of z0 tracking it).

Developed red-first: against the pre-fix code the test fails with a 36% relative z0 error at k_par = 2e-4 (got (-0.0983, 0.0129) vs want (-0.4543, 0.0599)), growing without bound toward the resonance.

Validation

• make — clean build
• ctest -E 'test_rhs_balance|test_periodic_convergence|test_periodic_vs_global' — 32/32 passed

Known red gates excluded as usual: the pre-existing test_rhs_balance baseline failure, the periodization-deformation gate, and the periodic-vs-global 5% gate — none touched by this change.

Supersedes #182, #183.