Layer 0
A cross-cutting data-readiness gate.
Layer 0 asks whether provenance, linkage, observation history, missingness, transformations, measurement, privacy, and uncertainty are adequately understood for the proposed use. It can be revisited whenever the question, population, outcome, data, time window, or method changes.
Concrete variables, rules, thresholds, and configurations belong to the applicable scientific implementation. They are not universal platform defaults.