Financial institutions across the country have begun preparing for a worst-case scenario if Congress fails to raise the nation’s borrowing limit, assembling plans to offer customers emergency loans or waive fees if their federal pay or benefits checks do not arrive on time. Navy Federal Credit Union, the largest in the country, said this week it would advance pay to direct-deposit members who are active-duty military or civilian employees of the Defense Department. It also said it would expedite approvals for lines of credit, overdraft programs and higher credit card limits.