//! Which files a search may return (RFC-060 §7).
//!
//! Plain data, shared by the query layer (`orbok-db` builds the SQL) and
//! the search engine (`orbok-search` fills it in from the UI's filters and
//! chosen folder). Both halves must agree, and a scope applied anywhere
//! but the query is not a scope: post-filtering shrinks the result set
//! below the requested limit and makes "no results" ambiguous
//! (RFC-041 §25.5).
/// One folder restriction: a registered source, and whether nested
/// folders count.
/// The scope of one search. Empty means unrestricted.