SERIES RELAUNCH: Ever since 1989, every episode of Agatha Christie’s Poirot, starring David Suchet as the persistent and persnickety Belgian detective, has been available on PBS’s Mystery! But for this 13^th and final season, the last three installments in the series, beginning today, are available only on the streaming network Acorn.TV – though, this fall, they’ll be made available to local public TV stations for rebroadcast. But for now, beginning with this episode – which features the last guest appearance by Zoe Wanamaker as Christie’s mystery-writing alter ego – you have to act like a squirrel, and hunt for Acorn. For my full review, visit NPR’s Fresh Air with Terry Gross later this afternoon.