A pair of Americans on Wednesday completed what had long been considered the world’s most difficult rock climb, using only their hands and feet to conquer a 3,000-foot vertical wall on El Capitan, the forbidding granite pedestal in Yosemite National Park that has beckoned adventurers for more than half a century.Tommy Caldwell and Kevin Jorgeson became the first to “free-climb” the rock formation’s Dawn Wall, a feat that many had considered impossible. They used ropes and safety harnesses to catch them in case of a fall, but relied entirely on their own strength and dexterity to ascend by grasping cracks as thin as razor blades and as small as dimes.The effort took weeks, as the two dealt with constant falls and injuries. But their success completes a years-long dream that bordered on obsession for the men.