Public Theater’s Free Shakespeare in the Park, the 67-year-old institution that is an annual signpost of summer — and this year, of a return to in-person life — for both the theater and for the civic fabric of New York City.When “Merry Wives,” a new version of the Bard comedy set in a South Harlem community of West African immigrants, begins performances in Central Park July 6, it arrives informed by all the tumult of the past 18 months.