It’s understandable that at age 70, Robin Byrd momentarily shows a rare melancholic side while reflecting on the inevitable spread and sag of a body once clad on television in only the skimpiest black crocheted bikini. But it’s triumphantly true to form that she almost instantly shrugs off those concerns and her clothes with them. She strolls down a Fire Island beach, sharing her generous unclad curves with the wind before climbing the stairs to her deck and blowing kisses to the Byrdwatchers, as her fans are known.  Bang My Box: The Robin Byrd Story  (surely a contender for year’s best title?) salutes a woman who for multiple generations of New Yorkers is as much a part of the city’s iconography as the Chrysler Building.…