Shift schema drift left: policy-aware compile-time contracts for typed JVM and Spark pipelines

Abstract: Schema drift in data pipelines is often caught only when a job touches real data. Typed-Dataset layers close part of this gap but require wholesale adoption; table-level enforcement systems close another part but operate at write time against a stored schema. We present a small Scala 3 framework that occupies the seam: it proves producer-to-contract structural compatibility under explicit policies at compile time, derives Spark schemas from the same contract types, and re-checks the actual DataFrame schema at the sink boundary before write. The artifact fuses the compile-time witness with a policy-aware runtime comparator that adds a nested-collection-optionality check Spark's built-in comparators omit and implements structural subset semantics for backward- and forward-compatible field sets. Evaluation covers compile-time proofs, runtime policy tests, builder-path end-to-end tests, and reproducible benchmarks on two environments. This is a narrow, honest mechanism artifact; the broader claim that compile-time structural contracts deliver measurable productivity or reliability in practice is stated as motivation and left for future work.