Everyone knows King Lear, the volatile monarch who tries to divide his kingdom between his three daughters and loses it all in the process. But seeing “Lear,” Marcus Gardley’s bold new adaptation of Shakespeare’s monumental tragedy, is enough to make you wonder whether you’ve ever really grasped the full impact of the original play. In its world premiere production at the California Shakespeare Theater, which opened Wednesday at the company’s Bruns Amphitheater in Orinda, this “Lear” is set in San Francisco’s Fillmore District during the 1960s, a heyday for Black art, music and culture, but also a time when Black displacement was draining the lifeblood of neighborhoods throughout the region.