A lightweight view of a Dataset, typically created via head() or tail().
This struct is public so it can be used in type signatures,
but its fields remain private to ensure data integrity.
Represents a dynamically-typed scalar value borrowed safely from a ColumnVector.
This acts as the single unified interface for extracting row-level data.
Mapping raw types to semantic types allows the engine to automatically
select the appropriate Scale (Linear, Temporal, or Discrete) and validation rules.