fw_tecs: Support tighter altitude tracking during low-height flight (#23519)

* fw_tecs: Support tighter altitude tracking during low-height flight. Added FW_T_THR_LOW_HGT defining low-height flight threshold

* tecs: Applied smoothed-out altitude TC transition to landings

* fw_tecs: modified tighter altitude control for low-height implementation

* addressed PR comments

* addressed PR comments

---------

Co-authored-by: Silvan Fuhrer <silvan@auterion.com>

src/lib/tecs/TECS.hpp

@@ -651,6 +651,14 @@ public:
 	float get_pitch_setpoint() {return _control.getPitchSetpoint();}
 	float get_throttle_setpoint() {return _control.getThrottleSetpoint();}
 
+	/**
+	 * Returns the altitude tracking time constant
+	 */
+	float get_altitude_error_time_constant() const
+	{
+		return 1.0f / math::max(_control_param.altitude_error_gain, 0.01f);
+	}
+
 	uint64_t timestamp() { return _update_timestamp; }
 	float get_underspeed_ratio() { return _control.getRatioUndersped(); }