On stage and off, you don't want to tangle with Joyce DiDonato - American soprano extraordinaire and practiced kickboxer too. The 45-year-old diva has been leaving audiences at the Royal Opera House in Covent Garden roaring for her singing and performing of the hugely demanding bel canto (beautiful singing) role of the doomed Queen Maria Stuarda - Mary Queen of Scots - in the second of Donizetti's three Tudor operas. There's hardly a more gripping and dramatic scene in opera than the one at the end of Act Two when DiDonato as Maria has a knock-down, drag-out confrontation with Queen Elizabeth I, sung by the up-and-coming Italian soprano Carmen Giannattasio. They spit insults at each other, DiDonato hurls "vil bastarda" (evil bastard) at her rival and pulls the tablecloth from under Elizabeth's picnic lunch, sweeping all the food and dishes to the floor - all this in the full knowledge that it will ensure she has her head chopped off. "I feel complet