Background-Consistent Residual Structure in Type Ia Supernovae

Recent analyses of Type Ia supernovae have attempted to resolve late-time cosmological discrepancies by extending or modifying the homogeneous background cosmological model. In this work, I instead constrain the background as tightly as possible and ask whether supernova residual structure persists. Using Pantheon+SH0ES supernovae together with DESI baryon acoustic oscillation constraints and joint CMB+BAO best-fit cosmologies, I show that an ordering-dependent residual statistic exhibits structure that cannot be absorbed by background parameter freedom. An ensemble of BAO-consistent background realizations produces an extremely narrow distribution of induced residual variation, far smaller than the observed effect. The result is robust to nuisance augmentation, duplicate collapse, and permutation tests, and is not explained by redshift, peculiar velocity corrections, or sky geometry. This work is intended as a methodological and diagnostic contribution. It constrains the explanatory power of homogeneous background cosmology with respect to supernova residual structure but does not propose a new physical model.