One covenant datapoint, self-describing.

Carries its own key/category/name inline so no second call to the
reference catalog is needed to interpret a search response. value is
heterogeneous by datapoint (string, number, bool); the required
value_type discriminator disambiguates it, as published in the
OpenAPI spec.

value is required and never null: a covenant entry whose source
datapoint yields no extractable value is dropped by the composition layer
rather than emitted as value: null alongside a value_type.
unit is omitted from the wire (response_model_exclude_none) unless
the value is a number with a unit.