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

drv/uart/uart_driver.h

@@ -10,34 +10,31 @@ enum class UartPort : uint8_t {
     MAX
 };
 
-struct UartConfig {
-    uint32_t baudrate = 115200;
-    uint8_t data_bits = 8;
-    uint8_t stop_bits = 1;
-    uint8_t parity = 0;
-};
-
-class UartBus {
+class USART {
 public:
-    explicit UartBus(UartPort port);
-    ~UartBus();
+    USART(UartPort port, uint32_t baudrate = 115200);
+    ~USART();
 
-    UartBus(const UartBus &) = delete;
-    UartBus &operator=(const UartBus &) = delete;
+    USART(const USART &) = delete;
+    USART &operator=(const USART &) = delete;
 
-    RetCode init(const UartConfig &config);
-    RetCode deinit();
     RetCode send_byte(uint8_t data);
     RetCode send_data(const uint8_t *data, uint32_t len);
     RetCode receive_byte(uint8_t *data, uint32_t timeout_ms);
-    RetCode irq_handler();
+    void reinit(uint32_t baudrate);
 
-    static UartBus &default_instance();
+    UartPort port() const { return port_; }
+    bool is_initialized() const { return initialized_; }
+
+    static void setDefault(USART *uart);
+    static USART *defaultInstance() { return default_; }
 
 private:
     UartPort port_;
-    UartConfig config_;
+    uint32_t baudrate_;
     bool initialized_ = false;
+
+    static USART *default_;
 };
 
 #endif