Function recv_select_publish_step 

pub fn recv_select_publish_step<Tx>(
    rxs: &mut [&mut dyn LaneRx],
    rr: &mut usize,
    tx: &mut Tx,
    buf: &mut [u8],
    on_full: OnFullV1,
    wait: &mut impl WaitStrategy,
) -> Result<bool, LaneTxError>
where
    Tx: LaneTx,

Receive from one of N RX lanes (round-robin) → publish.

This is a building block for fan-in roles (merge/join) without requiring allocs in the hot loop. The caller owns the round-robin cursor.