There’s a certain birthday math that kicks in around a certain age. You blow out the candles. Someone makes a joke about the fire marshal. You smile politely, eat the cake, and quietly accept that the “surprises” left in life are mostly of the medical variety. And then there’s Josephine, who turned 80 and decided the occasion called for dropping an original rap. She Didn’t Cover a Song. She Wrote One. Let’s be clear about…