Jem Aswad Senior Music EditorBruno Mars and Anderson .Paak, who announced last week that they’ve come together as a project called Silk Sonic, have released the first song from their forthcoming album, and it’s a glorious blast of vintage R&B called “Leave the Door Open” that could be a Best R&B Song Grammy winner from 1974.Even though the classic R&B sound the pair evokes is a decade older than they are, both the song and the arrangement are pitch-perfect, with swooning strings, honeyed