Bloomberg enjoys a close relationship with stock-market aficionados and financial stakeholders, particularly during the hours when stocks trade. Now the business-news giant is working to cultivate those ties at times when trading isn’t paramount. “Bloomberg This Weekend,” a three-hour broadcast that airs from 7 a.m. to 10 a.m. on Saturday and Sunday, will debut February 28. The show, hosted by David Gura , Christina Ruffini, and Lisa Mateo, will simulcast worldwide across Bloomberg’s audio,