Function build_vector_index 

pub fn build_vector_index(
    conn: &Connection,
    embedder_present: bool,
) -> Option<VectorIndex>

Build the in-memory VectorIndex from conn. When embedder_present is false, returns None (the keyword-only path doesn’t need an index). When the embedder is present but the DB is empty (or query errors), returns Some(VectorIndex::empty()) so write paths can populate it in-place.