fix(llcc68): align airtime estimate with radio config

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.

include/llcc68_definitions.hpp

@@ -14,7 +14,7 @@
 
 namespace app::driver::llcc68 {
 struct LLCC68;
-using airtime_t = std::chrono::duration<uint32_t, std::micro>;
+using airtime_t = std::chrono::microseconds;
 using error_code = std::error_code;
 
 enum class Errc : uint8_t {
@@ -1097,7 +1097,7 @@ struct transmit_result {
   error_code post_action();
 
   LLCC68 *self{};
-  std::chrono::milliseconds airtime_estimated;
+  airtime_t airtime_estimated;
 };
 } // namespace app::driver::llcc68