Many dance aficianados view Moscow’s Bolshoi Ballet as the gold standard by which all other companies must be judged. But even legends have to work to keep ahead of the pack, and the Bolshoi has been through some trying times in recent years as it tries to find its way through a world in which dance is evolving faster than ever, and “straight” ballet often gets short shrift. “Alexei Ratmansky is the latest artistic director to face the company’s basic quandary: how to introduce new work yet preserve the signature style and heritage of what Muscovites consider a national theater.”