In an effort to hold back the domestic effects of the global downturn, China is starting to spend hundreds of billions of dollars on new highways, railroads and other infrastructure projects. The stimulus plan, one of the world’s largest, promises to carry the modernity of China’s coasts deep into the hinterlands, buying the kind of great leap forward it took the United States decades — and a world war — to build, and priming China for a new level of global competition.