Calculate LoRa airtime from the exact LDRO value supplied to SetModulationParams instead of inferring it from symbol time.
Keep transmit_result airtime in microseconds for both LoRa and GFSK, using integer Hz math and 64-bit ceil division to avoid truncation errors.
Reapply modulation parameters during async TX flush so the estimator and the bytes sent to the radio stay in sync, and add a compile-time check for the intended SF7/BW125 implicit 6-byte airtime.