Robustness Tests of a Foundation-Only Low-Redshift Directional Residual in DES-SN5YR

This archive presents a robustness note and supporting diagnostic package for a DES-SN5YR Foundation-only low-redshift directional residual analysis. The primary subset is IDSURVEY=150 with zHD<0.060, containing 92 objects. The baseline covariance-aware directional residual fit gives Δχ²=14.945456, amplitude=0.101681 mag, and best-fit direction RA=20.366097°, Dec=57.880151°. The analysis tests whether the Foundation-only low-z feature is stable under internal robustness diagnostics, including random-sky nulls, residual sign-flip nulls, bootstrap and leave-k-out resampling, redshift splits, local-flow/VPEC cuts, velocity-frame sensitivity tests, leave-one-object influence, and exhaustive leave-two-out object-pair removal. The exhaustive leave-two-out test covers all 4,186 possible object-pair removals. No pair removal reduces Δχ² below 10 or shifts the fitted direction by more than 15°. The minimum leave-two-out Δχ² is 10.947866, the median is 14.740262, and the maximum direction shift is 10.795520°. AT2017zd is identified as the leading leverage-heavy object, but it is not result-defining. This archive is intended as an internal robustness and reproducibility package. It does not claim cosmological anisotropy, a preferred cosmic axis, URSF confirmation, independent object-level replication relative to Pantheon+, or elimination of local-flow/velocity-frame systematics. The strongest defensible interpretation is that the Foundation-only low-z residual is internally robust under the tested diagnostics, while its physical interpretation remains caveated by velocity-frame sensitivity, local-flow treatment, and complete low-z Foundation object overlap with Pantheon+. SHA256: 3f61f556960ecd4a3c9c5a2eee0ff560c88360b1ba93fee50162d5fb72a79e9b Prepared by Malin Hess with assistance from OpenAI’s ChatGPT for code-structuring, documentation, formatting, archive organization, visual generation, and wording review. Scientific interpretation and final responsibility remain with the author. All code is under MIT licensing.