WASHINGTON — Senate bargainers announced a bipartisan framework Sunday responding to last month’s mass shootings, a modest breakthrough offering measured gun curbs and bolstered efforts to improve school safety and mental health programs. The proposal falls far short of tougher steps long sought by President Joe Biden and many Democrats. Even so, if the accord leads to the enactment of legislation, it would signal a turn from years of gun massacres that have yielded little but stalemate in Congress. Leaders hope to push any agreement into law quickly — they hope this month — before the politic…