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 13th 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 &ndas