refactor: UartBus全面重构为USART，支持直接构造即可用

- UartBus -> USART (类名更符合外设命名规范)
- 构造函数 USART(UartPort port, uint32_t baudrate) 自动完成GPIO+外设初始化
- 移除default_instance()单例模式，移除init()/deinit()/irq_handler()
- 新增reinit(baudrate)支持时钟切换后波特率重新校准
- 新增static setDefault()用于printf路由选择
- 移除UartConfig配置结构体
- 更新iap.h/iap.cpp中UartBus引用为USART
- 更新main.cpp使用新USART构造模式

bsp/device/led/led_driver.h

@@ -7,7 +7,7 @@
 
 class Led {
 public:
-    Led(GpioBus &bus, uint32_t pin);
+    Led(GpioPort port, uint32_t pin);
     ~Led() = default;
 
     Led(const Led &) = delete;
@@ -19,8 +19,8 @@ public:
     void toggle();
 
 private:
+    GpioBus bus_;
     GpioPin pin_;
-    GpioConfig config_;
 };
 
 class LedManager {
@@ -37,17 +37,12 @@ private:
     LedManager();
     static constexpr uint8_t LED_COUNT = 4;
 
-    GpioBus bus_a_{GpioPort::A};
-    GpioBus bus_d_{GpioPort::D};
-    GpioBus bus_e_{GpioPort::E};
-    GpioBus bus_g_{GpioPort::G};
+    Led led0_{GpioPort::E, 0x08};
+    Led led1_{GpioPort::D, 0x80};
+    Led led2_{GpioPort::G, 0x08};
+    Led led3_{GpioPort::A, 0x20};
 
-    Led led1_{bus_e_, 0x08};   /* PE3 */
-    Led led2_{bus_d_, 0x80};   /* PD7 */
-    Led led3_{bus_g_, 0x08};   /* PG3 */
-    Led led4_{bus_a_, 0x20};   /* PA5 */
-
-    Led *leds_[LED_COUNT] = {&led1_, &led2_, &led3_, &led4_};
+    Led *leds_[LED_COUNT] = {&led0_, &led1_, &led2_, &led3_};
 };
 
 #endif