Test - Check that threaded perturbed-equilibrium loops match serial #479

Problem

CI never sets a thread count, so every @threads loop in PerturbedEquilibrium/SingularCoupling.jl, PerturbedEquilibrium/FieldReconstruction.jl and KineticForces/Compute.jl runs on one thread. The regression harness caches results by commit, not by thread count, so it cannot catch a race either.

Ask

• Add a test that runs a small multi-n PerturbedEquilibrium (PE) case at 1 thread and at more than 1. It should assert that the singular-coupling rows and the reconstructed fields are bit-identical. diiid_multi_n exercises the repeated-surface path, but a small Solovev multi-n PE case would be cheaper.
• Run that test in CI with more than one thread (e.g. JULIA_NUM_THREADS=2).

Also missing (could be split out)

PE currently stops only when force_termination is set. It should also refuse to run when:

• the plasma is unstable (et[1] < 0);
• the stability run used a wall (wall_settings is not nowall).

Follow-up to #334 and #477.