A schema + range + enum data contract with drift detection for tabular and sensor (rosbag) batches. It catches the batch that still parses but whose meaning changed - a firmware unit switch (Celsius to Fahrenheit), a new category, a sensor dropout - exactly what a naive did-it-parse gate ships. On the committed demo batch: the naive check reports 0 issues; the contract reports 36 violations plus drift (joint_temp mean 45 to 112, z=14.9) on the same data. Runs as a CI gate and names the cause, not the symptom; the clean batch passes untouched. Public code: github.com/jigonyoo/data-contract-guard