Of all the shows that could have released a Christmas special this year, perhaps the least likely candidate was HBO’s “Euphoria.” For all its glitter, Sam Levinson’s decidedly nihilistic Gen Z drama is hardly the pinnacle of cheer. In the show’s hypnotic, chaotic first season, its teenage ensemble grappled with substance abuse, violence, codependency, and rape culture. Now it’s back – temporarily – for a two-part special meant to bridge the gap between seasons one and two. Part one, “Trouble Don’t Last Always,” introduces a more subdued and contemplative chapter in this already accomplished narrative.