Function mlkem_add_mod_q_w16 

pub fn mlkem_add_mod_q_w16(
    a: &[Word16],
    b: &[Word16],
) -> Result<LogosSeq<Word16>, String>

(a + b) mod q, element-wise on Word16 [0, q) — the native modular-add the Logos matrix-vector loop calls (so the orchestration never does Word16 arithmetic, only structure).