Function airtime_ms 

pub fn airtime_ms(
    sf: SpreadingFactor,
    bw: Bandwidth,
    payload_bytes: usize,
) -> u32

Conservative upper bound on LoRa on-air time in milliseconds.

Uses the standard LoRa airtime formula: explicit header, CRC on, CR 4/5, auto-LDRO. Call this with MAX_PAYLOAD to get the worst-case figure for t_frame_ms.