As Hillary Clinton assailed Donald J. Trump on Thursday for fanning the flames of racism embraced by the “alt-right,” the community of activists that tends to lurk anonymously in the internet’s dark corners could hardly contain its glee. Mrs. Clinton’s speech was intended to link Mr. Trump to a fringe ideology of conspiracies and hate, but for the leaders of the alt-right, the attention from the Democratic presidential nominee was a moment in the political spotlight that offered a new level of credibility. It also provided a valuable opportunity for fund-raising and recruiting.