LOS ANGELES — Nominees Jessie Buckley and Rose Byrne led the parade of entertainment luminaries arriving at Sunday’s Oscars, the film industry’s highest honors, for an unusually open best-picture race that pits vampire hit "Sinners" against the darkly comic thriller "One Battle After Another." Security for the ceremony was tight as some of the biggest names in Hollywood walked a red carpet decorated with trees to evoke the feeling of a Zen garden. Designers said they hoped the scene, which also adorned the stage, would deliver a feeling of calm in a chaotic world. Organizers said they were working closely with the FBI and Los Angeles police after a federal warning of a possible Iranian threat against California, though authorities have cited no specific or credible danger to the Academy Awards. Attendees had to cross through several traffic checkpoints and go through metal detectors to make their way to the Dolby Theatre. Hosted by Conan O’Brien for a second year, the festivities will feature a wide-open contest led by "Sinners" with 16 nominations — a record number in the nea