For years, fast-food chains fought to steal customers from one another. Increasingly, their biggest competition may be coming from the grocery aisle.As inflation-weary consumers rethink what counts as a good value, supermarkets’ expanding prepared-food offerings are becoming a viable alternative to a drive-thru meal. New foot-traffic data from location analytics firm Placer.ai suggests that the shift is beginning to show up in Americans’ choices about where to eat.Restaurant traffic has softened overall this year, but…