Could you please try to describe what is the API/interface that a cell module currently must implement (with contract/constraints required of each function)? I tried to read shell.lua and synth.lua, but I wasn't able to understand them bacause I'm not sure what interface they're trying to implement... :,( it would be really helpful! Please don't worry if "it may change in future", just if you could put down current status quo, simply to help me/others understand? Thanks!