The downing of a Russian warplane by Turkey threatened to lead to a wholesale breach in the countries’ relations on Thursday, with the Kremlin preparing to sever economic ties and Turkish officials saying they had no reason to apologize. Prime Minister Dmitri A. Medvedev of Russia gave government officials two days to draw up a list of ways to curb economic links and investment projects. That included the possible shelving of a multibillion-dollar deal to build a gas pipeline through Turkey that President Vladimir V. Putin had trumpeted as a welcome alternative route for Russian gas exports to Europe. Mr. Putin and his Turkish counterpart, Recep Tayyip Erdogan , stoked the confrontation by hurling insults at each other and demanding redress. […]