Elon just disclosed a $900 billion stake in SpaceX. Billions of those shares only become his if a million people live on Mars. The filing shows 6.42 billion shares, 48.4% of the company. But a huge chunk is restricted stock and options tied to vesting milestones, and the milestones escalate fast. Start with the tame ones. Some tranches require SpaceX to clear valuation targets in the trillions. The company IPO'd two months ago at $2 trillion, off the largest IPO in history at $85.7 billion…