MOSCOW Russia said on Sept. 2 that it had signed a legally binding agreement with China to build a new cross-border gas pipeline , the latest in a series of moves by the allies to bolster ties. The memorandum of understanding to build the Power of Siberia 2 pipeline was signed during a visit to China by Russian President Vladimir Putin. Although the move is a step forward in the long negotiations on the massive pipeline, the memorandum is not definitive, and many details, such as pricing,