Full Rephasing Collapses the Common Singlet Relative Sign
The selected diagonal parity is not the full symmetry of the common-singlet messenger incidence. Exact enumeration gives a 64-element sign-rephasing group whose projection contains all four sign changes of the down entrance and singlet vacuum signs.
A legal down-sector rephasing flips the down entrance while holding the
singlet fixed, moving between the proposed relative classes. Tree elimination
contains two messenger denominators and hence only the square of the singlet.
The remaining sign of the down Yukawa is removed by a right-handed quark
rephasing, leaving physical16 unchanged.
The first nonfaithful arrow is the passage from the selected diagonal subgroup to the full source rephasing groupoid. A viable reference needs a closed rephasing-invariant interference cycle or a genuinely new external experiment.
Evidence: research/flavor/flavor-common-singlet-relative-sign-collapse.md
and research/flavor/results/wp633_common_singlet_relative_sign_collapse.json.