Cosmic-string background benchmarks (Sandbox Physics, AC83)

Contents
  scripts/     six Python files, standard library only
  benchmarks/  inputs.json, outputs.json and failure-cases.json from the saved run

Run from the scripts folder so the main script can import its siblings:
  cd cosmic-string-background/scripts
  python3 reproduce_benchmarks.py ../my-run

The saved run used Python 3.9.6 and took 37.4 seconds.

Units: time and length in 1/H0 with c = 1, frequencies in Hz, densities as h^2 Omega_gw.
Model: one-scale Nambu-Goto loops (Model I of Auclair et al. 2020, arXiv:1909.00819v1),
constant number of particle species, radiation and matter loop parameters blended by the
radiation share of the energy density. This is a teaching model; it fits no data.

Two printed values in Auclair et al. are not reproduced and are recorded as unresolved in
outputs.json: the eq. 6.1 and 6.2 prefactor (printed 1.02e-2, symbolic 1.0146e-3) and the
6.04e-14 plateau quoted after figure 8. Neither is corrected.
