If you only watch the first half of La La Land (just released on DVD), you’ll find yourself swept into the blue-sky, sunshine, and citrus colors of Hollywood dreams. The opening big production number on the jammed LA freeway introduces us to Mia (Emma Stone) and Sebastian (Ryan Gosling), off to a prickly start but eventually falling in love and cheering each other on. He wants to revive the lost art of pure jazz in his own club; she wants to become a famous Hollywood actress.   Part one concludes with Seb and Mia’s Boy-Gets-Girl kiss in an iris fade so typical of bygone Hollywood Happy Endings. But, as in…