//! Pluggable per-item validation seam for the collection engine.
//!
//! The engine treats validation as an injected policy:
//! a [`Validator`] inspects each item after its transforms and rejects it with a typed error.
//! The engine ships no built-in validator —
//! callers opt in (for example a schema-backed validator for tabular records).
use AppResult;
use crateDatasetItem;
/// Rejects invalid items before they are materialized by a sink.