🎇 `cib::service` pointers should be `constinit` initialized to a default function pointer or type

[lukevalenty]

There are some unfortunate cases in which a cib::service pointer may not be initialized. If it is later used, UB is invoked and it becomes very difficult for a library user to understand what is going on. This happens if nexus.init() is never called, or if a non-existent service is called through cib::service.

We can't check this at compile-time, but we can ensure that each cib::service pointer dereferenced is constinit initialized to a default implementation that logs an error when called. For the flow library, this would simply be a function pointer that logs an error saying the given flow was never added to the nexus.