Historical practices can help define the separation of powers. One branch’s claim of authority and another branch’s acquiescence can put a “gloss” on the sparse text of Articles I–III, especially when repeated over time. . . . [W]e try to explain why courts were on the sidelines of these gloss games and why other disputes over Article III are likely to reflect a similar structure. The key, we think, lies in a distinctive feature of judicial decision making: the doctrine of stare decisis.