Function build_view_core 

pub fn build_view_core(
    actors: &[Actor],
    contacts: &[ChatContact],
    assignments: &BTreeMap<String, Vec<String>>,
    categories: Vec<DirectoryCategory>,
    agreements: &[Agreement],
) -> DirectoryView

Pure core: merge directory actors + chat contacts into one categorised view, given the persisted category assignments and the category list. Kept pure (no filesystem) so it is deterministically testable; build_view wires it to the persisted stores.