Function search_core 

pub fn search_core(
    entries: Vec<DirectoryEntry>,
    categories: Vec<DirectoryCategory>,
    query: &str,
    selected: &BTreeMap<String, Vec<String>>,
) -> DirectorySearchResult

Pure search core: rank entries by a concept-expanded query and narrow by selected facets, returning the entries plus drill-down facet counts. Kept pure (no filesystem) for deterministic testing.