For decades, people have fretted about the financial sustainability of America’s bedrock retirement income program. Now, Social Security’s precarious fiscal state is an issue for the here and now.How to fix it is set to be a defining political fight of the next several years, with millions of Americans’ benefits and the fiscal future of the United States at stake.Why it matters: There is a tug-of-war between the benefits future retirees receive and the taxes that working-age people pay…