The width and driver index of one port: what a wave needs from the BUILD, and nothing else. Two ints, because this is what travels when the simulator is in another process. Everything else a selector shows - names, labels, which ports carry a wave at all - is a fact about the design, which the renderer holds; only elaboration facts need the simulation, and these are they. The driver index doubles as the build's read handle: `SimRead` accepts it for as long as this build lives, and the next build invalidates it wholesale.