Function lora_airtime_ms 

pub fn lora_airtime_ms(
    sf: u8,
    bw_hz: u32,
    cr_denom: u8,
    payload_bytes: usize,
) -> u32

Conservative LoRa on-air time estimate in milliseconds.

Standard LoRa airtime formula with explicit header, CRC on, and auto-LDRO; the preamble of 8 + 4.25 symbols is approximated as 12. Numeric twin of umsh_radio_loraphy::airtime_ms (which takes lora-modulation enums), generalized over the coding-rate denominator (5 for CR 4/5 through 8 for CR 4/8).

Out-of-range inputs are clamped rather than rejected: this is an estimate for scheduling and duty accounting, not a validator.