Harvard University’s investment arm disclosed a $2.2 billion stake in SpaceX, revealing a massive payoff from an early investment in Elon Musk’s rocket company following its blockbuster public debut. Harvard Management Company reported the position in a regulatory filing Friday, making SpaceX the largest individual stock holding disclosed in its $4.3 billion portfolio of U.S. equities. The investment highlights how SpaceX’s record-setting June i…