//! Categories of document-defined IDs, mapped from schema `$ref` names.
/// A category of ID that a value position may reference, derived from the
/// schema type of the value (its `$ref` name).
///
/// For example, an `EdgeGroup`'s `things` array has items `{ "$ref":
/// "#/$defs/ThingId" }`, so completing one of its values should offer the
/// [`IdCategory::Thing`] IDs already defined in the document.