The Cepheid metallicity term has been litigated for thirty years, with published values spanning negligible to 0.3 mag/dex and every revision carrying freight on H₀. The audit has narrowed the room: JWST confirmed the photometry, infrared analyses suppress the sensitivity, and metallicity-independent rungs agree on distances at the percent level. Spanning the 5.6 km/s/Mpc tension with chemistry now requires an error several times any surviving estimate, applied conspiratorially across anchors.
The model needs the ladder to be wrong, because one global H₀ leaves no other exit, so every percent-level calibration uncertainty gets audited as a candidate culprit for a five-percent discrepancy. The metallicity term is the longest-serving suspect, interrogated past the point where the arithmetic could convict it.
SCT has no need for the Cepheids to be miscalibrated, and the audit's trajectory is its evidence. The framework requires the local measurement to be correct: calibrators inside the KBC volume sample a genuinely enhanced expansion (P17, P19), so the ladder reading 73 while the CMB reads 67 is two true measurements of an inhomogeneous Λ_eff field, not a chemistry error awaiting discovery. Every cross-check that vindicates the Cepheids, JWST's clean photometry, the percent-level distance agreement with TRGB and JAGB in shared hosts, the failure of any metallicity revision to move H₀ more than fractionally, is the environmental reading's prediction coming in: the rungs are sound because the signal was never in the rungs.
The framework even accommodates the residual method spread honestly: where Cepheid-anchored and TRGB-anchored H₀ differ by 2 to 3 km/s/Mpc despite agreeing on distances, the difference rides on which supernova hosts and environments each calibrator set weights (the registered frame-tree and environment offsets of P10), structure the one-number framework must call bias and SCT reads as sampling. Thirty years of metallicity litigation ends with the chemistry acquitted and the geography indicted. Foundations in Paper 1 (P17, P19) and the frame-tree treatment of Paper 2, From Chaos To Common Ancestry.
No premises are spent on this entry at all: it is the Hubble mechanism's consistency check, passed.
The acquittal is reversible: a future calibration demonstrating a coherent metallicity bias large enough to bring Cepheid H₀ to 67-68, surviving the JWST photometry and the multi-rung distance agreement, would restore the systematic explanation and remove the environmental signal SCT requires. Conversely the environmental reading keeps its own kill: no void-versus-cluster H(z) difference at the 2 percent level in DESI-class data guts the mechanism that makes the ladder's 73 physical.