Governors and mayors are ratcheting up mask mandates and imposing restrictions on small indoor gatherings that have been blamed for accelerating the spread of the coronavirus. But such measures are largely unenforceable, and officials are banking on voluntary compliance. Experts say that if people disregard the new state and local restrictions and socialize anyway, that could put greater stress on overburdened hospitals and lead to an even bigger spike in sickness and death over the holidays.