ForceFreeStates: ex5 kinetic-damping imag(et[1]) pin (rtol=0.08) is platform-fragile (fails on macOS aarch64) #273

Summary

The regression_solovev_kinetic_calculated (ex5) fullrun assertion in test/runtests_fullruns.jl

@test imag(et[1]) ≈ -0.711 rtol = 0.08   # kinetic damping rate

passes on the Linux x86 CI baseline (where -0.711 was tuned) but fails on macOS aarch64, which produces imag(et[1]) = -0.8556 (~20% off, outside the rtol=0.08 band [-0.768, -0.654]). The well-conditioned real(et[1]) ≈ 15.888 (rtol=0.01) passes on both platforms; only the FP-sensitive imaginary (kinetic-damping) part diverges.

Why

imag(et[1]) here is a kinetic damping rate — a small, FP-reassociation-sensitive quantity. This is the same class of cross-platform fragility we already characterized for the kinetic multi-n et[1] (a near-marginal eigenvalue): @inbounds @simd floating-point reassociation differs between platforms (aarch64 vs x86) and between --check-bounds=auto (dev) and --check-bounds=yes (Pkg.test/CI), and a marginal/damping quantity amplifies that ~0.1% into a 20%+ swing. The underlying kinetic-DCON physics is the same on both platforms (validated <2% vs Fortran on the DIIID benchmark, PR #112).

Scope

This is a develop-side test-robustness issue, not a regression from any feature branch — the kinetic-eigenvalue code is unchanged. It was surfaced while merging develop into feature/tearing-growthrates: every other suite entry passes on macOS; only this one assertion fails.

Suggested fix

Make the imag(et[1]) assertion platform-robust, consistent with how the multi-n et[1] pin was handled: keep the tight pin on the well-conditioned real(et[1]) ≈ 15.888, keep the sign check (imag(et[1]) < 0, kinetic damping), and widen the magnitude tolerance on imag(et[1]) to cover both the Linux (-0.711) and macOS aarch64 (-0.856) values (e.g. a bracket such as -1.0 < imag(et[1]) < -0.5), with a comment documenting the platform/FP sensitivity. The well-conditioned real part remains the real regression anchor.

Reference: test/runtests_fullruns.jl ex5 block; failing value imag(et[1]) = -0.8555633612323581 on macOS aarch64 (Apple), Pkg.test (--check-bounds=yes).