AARON ROSS IN 30 years as Chad's president, Idriss Deby emerged as one of Africa's wiliest political survivors, holding on to power through rebellions that reached as far as his palace gates and establishing himself as a key military ally of Western powers. But it was in an apparent moment of triumph, just hours after he was declared the winner of an April 11 election, that the 68-year-old former army officer met his end, killed in battle fighting rebels in the desert north. Deby had ruled for three decades by centralising power around his family and Zaghawa ethnic group. Key…