By Sophie Yu and Norihiko Shirouzu BEIJING (Reuters) -China staged a low-key but historic debut of its C919 civil jet at its biggest air show on Tuesday, with some delegates unable to attend the scaled-down event because of Beijing’s zero-COVID policy as cases reached the highest level in six months. In a reminder of the continuing health crisis slowing China’s return to global aviation, organisers of Airshow China in the southern city of Zhuhai had urged attendees to arrive three days early because of COVID-19 precautions. Even then, some were blocked from joining day one because they had vis…