fix: 修复串口 read timeout 未生效的问题

- 将串口文件描述符设置为非阻塞模式，确保 read() 不会阻塞
- 修改 VMIN 从 1 改为 0，让 select() 正确控制读取超时
- 添加 Timeout 错误代码，超时时正确返回错误而不是 (0, nil)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

serial_unix.go

@@ -119,7 +119,8 @@ func (port *unixPort) Read(p []byte) (int, error) {
 			if port.readTimeout == NoTimeout {
 				continue // 无超时设置，继续等待
 			}
-			return 0, nil // 返回0字节表示超时
+			// 超时应返回 Timeout 错误，而不是 (0, nil)
+			return 0, &PortError{code: Timeout, causedBy: unix.ETIMEDOUT}
 		}
 
 		n, err := unix.Read(port.handle, p)
@@ -352,7 +353,9 @@ func nativeOpen(portName string, mode *Mode) (*unixPort, error) {
 		return nil, &PortError{code: InvalidSerialPort, causedBy: fmt.Errorf("error configuring port: %w", err)}
 	}
 
-	unix.SetNonblock(h, false)
+	// Set non-blocking mode to ensure read() doesn't block after select() returns
+	// The select() call provides the timeout mechanism, not the read() call
+	unix.SetNonblock(h, true)
 
 	port.acquireExclusiveAccess()
 
@@ -513,7 +516,14 @@ func setRawMode(settings *unix.Termios) {
 	settings.Oflag &^= unix.OPOST
 
 	// Block reads until at least one char is available (no timeout)
-	settings.Cc[unix.VMIN] = 1
+	// VMIN=1: Block until at least 1 byte is available
+	// VTIME=0: No inter-character timeout (not used with VMIN=1)
+	//
+	// NOTE: When readTimeout is set, the select() system call provides the timeout
+	// and VMIN/VTIME settings are not used for timeout purposes.
+	// However, VMIN=1 can cause issues with select() on some platforms/
+	// terminal drivers. Using VMIN=0 allows select() to work reliably.
+	settings.Cc[unix.VMIN] = 0
 	settings.Cc[unix.VTIME] = 0
 }