Adam B. Vary Senior Entertainment Writer SPOILER ALERT: This story includes plot points in Episode 4 of “The Lord of the Rings: The Rings of Power,” currently streaming on Amazon Prime Video, as well as one major story development written by author J.R.R. Tolkien in the appendices of “The Lord of the Rings.” Like so many of her castmates of “The Lord of the Rings: The Rings of Power,” Cynthia Addai-Robinson didn’t know exactly who she was playing until she arrived in New Zealand to shoot the Amazon Prime Video series. Unlike several of her castmates, however, Addai-Robinson was not an obsessive fan of the expansive world of Middle-earth created by author J.R.R. Tolkien. But once she was cast as Míriel, the Queen Regent of the resplendently prosperous island nation of Númenor, the veteran actor (“Power,” “Shooter,” “Arrow,” “Spartacus”) dove in head first into the rich history of Númenor and Middle-earth.