FeatureSchema

One feature-extraction contract of this engine: the id a model manifest names, the column layout that id promises, and the extractor that produces a row.

This is what makes a manifest's featureSchema field actionable rather than documentation. A host that loads a model no longer hardcodes which extractor to pair with it — the artifact declares its schema, this registry resolves it, and a model trained on a different layout fails to load instead of silently receiving a vector in which every column means something else. That failure mode is the reason the field exists: two schemas of equal width (there is no such pair today, but nothing prevents one) would otherwise be indistinguishable at run time.

The engine is the single source of truth for the values themselves — every extractor here is the same code the training enrichment calls — so train == serve holds by construction rather than by a Python reimplementation.