“Like all securities, an economic arrangement can qualify as an investment contract only if it involves an ongoing business enterprise whose management owes enforceable obligations to investors. Absent such obligations, the contract is just an asset sale,” Coinbase argued. “Because no such obligations are carried in the transactions over Coinbase’s secondary market exchange, and because the value that Coinbase purchasers receive through these transactions inheres in the things bought and traded rather than in the businesses that generated them, the transactions are not securities transactions.”