fw pos ctrl: refactor takeoff mode

- post takeoff, the aircraft follows the infinite line sourced from the launch point in the direction of the takeoff waypoint
- takeoff waypoint altitude is used as a clearance altitude, set such that once above, the aircraft has cleared all ground occlusions and may proceed with the mission
- runway takeoff state machine simplified to throttle ramp, clamped to runway, climbout, and fly
- throttle ramp must complete before switching to next state to avoid a jump in throttle setpoint just after takeoff if the takeoff airspeed is reached before the ramp is complete
- roll constraints near ground post takeoff removed from runway takeoff class (handled externally now)
- minimum airspeed in TECS is reduced to takeoff speed (if necessary) to lower the underspeed detection bound
- lateral-directional guidance uses a different period parameter during ground roll

src/modules/navigator/mission_block.cpp

@@ -182,6 +182,13 @@ MissionBlock::is_mission_item_reached()
 				_waypoint_position_reached = true;
 			}
 
+		} else if (_mission_item.nav_cmd == NAV_CMD_TAKEOFF
+			   && _navigator->get_vstatus()->vehicle_type == vehicle_status_s::VEHICLE_TYPE_FIXED_WING) {
+			/* fixed-wing takeoff is reached once the vehicle has exceeded the takeoff altitude */
+			if (_navigator->get_global_position()->alt > mission_item_altitude_amsl) {
+				_waypoint_position_reached = true;
+			}
+
 		} else if (_mission_item.nav_cmd == NAV_CMD_TAKEOFF
 			   && _navigator->get_vstatus()->vehicle_type == vehicle_status_s::VEHICLE_TYPE_ROVER) {
 			/* for takeoff mission items use the parameter for the takeoff acceptance radius */