CDN-213: The Holdout Residual

CDN-213 audits the strict leave-one-out residual gate after CDN-212. The public Agol et al. 2024 source bundle contains the manuscript source, bibliography, 7534-row full-fit forecast CSV, and figure PNG. The manuscript describes a drop-one procedure and reports uncertainty inflation by 3.14, but the dropped-transit forecasts, per-transit leave-one-out residuals, and reproduction notebook/code are not packaged. Full-fit residuals remain diagnostic only: max residual 50.530 s, max published |z| 1.302, diagnostic de-inflated max |z| 2.731. TTY candidate promotion remains blocked.