Function make_stub_idempotent 

fn make_stub_idempotent(
    wstore: &Store,
    def_id: &str,
    name: &str,
    now: i64,
) -> Result<(String, bool), String>

Returns (stub_id, newly_inserted). newly_inserted = false when the UNIQUE constraint fires (stub already existed); callers use this to avoid broadcasting agents:changed on no-op calls.