Contributed by Jonathan Stevenson / Celebrating the 250th anniversary of a country putatively vaunted for its liberal pluralism when in fact it is nearly as bitterly divided as it was during the Civil War has been a fraught affair, by turns sheepish, rueful, and obtuse. It is difficult to find a cultural sweet spot in the absence of a palatable center. Waterbury’s Mattatuck Museum rises to the challenge with the portrait exhibition “About Face,” which spans the eventful quarter-millennium and presents multiculturalism not as a political accident subject to debate and correction but as a rooted and immutable structural feature of the American polity. Impeccably curated by Rebecca McNamara – the museum’s chief curator – in her inaugural effort, the show is a triumph of sensitivity and finesse.