Function select_engine 

pub fn select_engine<'a>(
    kind: GraphEngineKind,
    conn: &'a Connection,
    _db_path: &Path,
) -> Result<Box<dyn GraphEngine + 'a>>

Open the requested engine. The cozo variant returns an error unless the cozo Cargo feature is enabled. We deliberately do not silently fall back to SQLite — if the user asked for Cozo, they should see the gap.