The Mariners didn’t know they were trading for immortality on May 25, 1989. They knew they were trading their best pitcher for possibility. On that day, the Mariners sent Mark Langston to the Montreal Expos and got back three young pitchers: Randy Johnson, Brian Holman and Gene Harris. It’s almost comically undersized compared to what the deal became. What Seattle really acquired wasn’t just a 25-year-old left-hander with control problems. The Mariners acquired the beginning of one of baseball’s most intimidating transformations. It feels worth revisiting as the Mariners celebrate Johnson and retire No. 51. Before the towering mythology of the Big Unit, there was a deal nobody could fully understand in real time. Montreal thought it was getting the safer answer for a pennant push. Seattle, whether by patience, scouting conviction or a little bit of luck, got the arm that would help change the franchise’s ceiling. Johnson wasn't polished when he arrived in Seattle.…