Make multilevel-atom test independent of FP contraction (#1426) - #3319
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At resolution 40 the optical period spans only ~13 time steps and the laser spikes chaotically instead of settling. Between spikes the field collapses to ~1e-18 and the laser regrows out of that remnant, so the sampled value depends on whether the compiler contracts a*b+c into an FMA: arm64 does by default, x86 does with -march=native. Raise the resolution to 200, where the laser reaches a steady state, and assert the steady-state envelope at the cavity boundary instead of an instantaneous field value. Steady-state lasing fixes the amplitude but not the phase, so the envelope is reproducible: contracted and uncontracted builds agree on it to 6e-6, against 98% before. The envelope no longer constrains the lasing frequency, which the old sample caught through its phase in builds that contracted the same way. A frequency check can be added later if that coverage is wanted, the peak of a DFT of the boundary field is identical across both builds.
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At resolution 40 the optical period spans only ~13 time steps and the laser spikes chaotically instead of settling. Between spikes the field collapses to ~1e-18 and the laser regrows out of that remnant, so the sampled value depends on whether the compiler contracts a*b+c into an FMA: arm64 does by default, x86 does with -march=native.
Raise the resolution to 200, where the laser reaches a steady state, and assert the steady-state envelope at the cavity boundary instead of an instantaneous field value. Steady-state lasing fixes the amplitude but not the phase, so the envelope is reproducible: contracted and uncontracted builds agree on it to 6e-6, against 98% before. The envelope no longer constrains the lasing frequency, which the old sample caught through its phase in builds that contracted the same way. A frequency check can be added later if that coverage is wanted, the peak of a DFT of the boundary field is identical across both builds.