Addition of Phi_res_crit_callen

[priyanshlunia]

Closes #240.

Summary

Adds the Callen critical resonant flux — the vacuum resonant flux Phi_res_v at which the vacuum
island width equals the Callen critical width (w_vac = w_v_crit) — to gpout_singfld
(gpec/gpout.f). This is the Callen-model analog of the SLAYER Phi_res_crit already output by
GPEC, enabling a Callen-vs-SLAYER threshold comparison (the motivation in #240 / #237).

What it does

The vacuum island scales as w_vac ∝ sqrt(Phi_res), so the flux required to reach the critical
width is

Phi_res_crit_callen = |Phi_res_v| * (w_v_crit / w_vac)^2      [Tesla]

which reuses the existing Callen machinery (hw_v, hw_v_crit, vsingfld) — both widths are in
psi_n at the point of use, so the ratio is clean. The |Phi_res_v| magnitude cancels
analytically, making this a true, drive-independent threshold (verified below).

Outputs added

Mirrors the existing b_crit / Phi_res_crit plumbing, so it appears alongside the other Callen
quantities in all three places:

Output	Name
netCDF gpec_profile_output_n*.nc	Phi_res_crit_callen (units T)
ASCII gpec_singfld_n*.out	Phi_crit_callen column
verbose stdout table	Phi_crit_cal column

Verification (DIII-D ideal example)

• Builds cleanly (gfortran); baseline Phi_res_crit_callen = [8.74e-4, 1.01e-3, 7.33e-4, 8.18e-4] T
  across the four rational surfaces — consistent across netCDF, ASCII, and stdout, and comparable in
  magnitude to SLAYER's Phi_res_crit (the intended cross-model comparison).
• Validation: scanning the coil current (0.5×–8×) shows Phi_res_crit_callen stays exactly
  constant over the full 16× drive range (a genuine drive-independent threshold), while the measured
  w_vac (∝√Φ) crosses the constant measured w_v_crit precisely at Phi_res_v = Phi_res_crit_callen.
  Normalized across all surfaces, every curve collapses onto y = √x through (1, 1). The measured
  widths come from GPEC's own island-width computation while the threshold comes from the new ratio
  formula, so their coincidence is an independent check.

Notes

• Single-file change (gpec/gpout.f, gpout_singfld); no changes to inputs or the shipped example.
• The ASCII gpec_singfld row/header widths were incremented for the new column.