Why a contract
Modules that share everything eventually corrupt each other.
The lazy integration between two workbooks is total visibility: let each read whatever it wants from the other. It's also how module boundaries dissolve. If the throttle could reach into the console's raw ledgers, it would inevitably start consuming numbers it has no authority to interpret — floating P&L, unsettled outcomes, half-closed cycles — and the interpreter doctrine would erode one convenient lookup at a time. The handoff contract replaces total visibility with a defined interface: a short list of settled, authority-bearing figures that cross the boundary at a defined moment, in one direction at a time. Everything on the list is guaranteed meaningful; everything off the list is structurally invisible. The boundary does the discipline so the operator doesn't have to.
