Replace Xbox joystick control with RadioMaster Pocket + XR1 RC receiver

Migrates the 4WD control input from SDL2/Xbox gamepad to a serial RC
receiver per doc/joystick.md's functional spec: CH1 steering, CH2
throttle, CH5 emergency brake (highest priority), CH7 speed mode
(0.3/0.6 m/s), with a 200ms fail-safe that forces an immediate stop
on signal loss. Channel mapping, deadzones, port, and baudrate are
all configurable via --rc_* CLI flags. LiDAR avoidance, jerk-limited
motion profile, and wheel-fault detection are unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

run_4wd.sh

@@ -23,9 +23,9 @@ if [ ! -f "./xbox_motor_control_cpp" ]; then
 fi
 
 echo "=================================================================="
-echo " [시작] C++ 100Hz 초저지연 4WD 조이스틱 + 라이다 장애물 회피 시작 ($PORT)"
-echo "  - 라이다 기능 토글: Xbox 컨트롤러 X 버튼"
-echo "  - 정지/종료: B 버튼 또는 Ctrl+C"
+echo " [시작] C++ 100Hz 초저지연 4WD RC(RadioMaster Pocket + XR1) + 라이다 장애물 회피 시작 ($PORT)"
+echo "  - 라이다 기능 비활성화: --no_lidar 옵션"
+echo "  - 비상 정지: CH5 브레이크 스위치 또는 Ctrl+C"
 echo "=================================================================="
 
 # 3. C++ 4WD 프로그램 실행 (추가 인자 전달 가능)