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.go

@@ -163,6 +163,8 @@ const (
 	ErrorEnumeratingPorts
 	// PortClosed the port has been closed while the operation is in progress
 	PortClosed
+	// Timeout the read operation timed out
+	Timeout
 	// FunctionNotImplemented the requested function is not implemented
 	FunctionNotImplemented
 )
@@ -192,6 +194,8 @@ func (e PortError) EncodedErrorString() string {
 		return "Could not enumerate serial ports"
 	case PortClosed:
 		return "Port has been closed"
+	case Timeout:
+		return "Read timeout"
 	case FunctionNotImplemented:
 		return "Function not implemented"
 	default: