By Howard Beck The Nets star is fed up, and two factors appear to have led to his unhappiness. NEW YORK – On a late December evening in 2020, when the state of the world was bleak, Kevin Durant slipped on a Nets jersey and stepped into the Brooklyn spotlight for the first time. The possibilities seemed infinite. Hope sprang eternal. Durant was brilliant that night—because, well, Durant is nearly always brilliant when he takes the court—but his long-awaited Nets debut was strange, strained, muted, and performed as it was in a near-empty arena, in the midst of a global pandemic. As omens go, thi…