GERT Historical Lab: reproducible grid risk calculations from 216 hours of public ERCOT data

A citable snapshot of the GERT Historical Lab teaching exercise (https://gert-d.vercel.app/history): a deterministic, fully checksummed reproduction of a grid risk calculation from 216 hours of public ERCOT native-load observations (three 72-hour windows: Aug 9-11 2023, Apr 10-12 2023, Feb 14-16 2021). Includes the methodology document, the bundled dataset with SHA-256 provenance, and an independent Python-standard-library checker that recomputes every hourly value. Also includes the public decision record rejecting the probabilistic research candidate ercot-delta-quantile-v1.4 for production use after it missed its predeclared calibration gate. This is a teaching exercise, not a validated forecast.