It's now public

So I have to convert the Expectation initializers with “#expect” calls, right.

It’s the latter.

So, the same methods as Cocoa apps?

Where in the process do you insert the custom dictionary? Besides classic app formats, SwiftUI also supports UI/NSApplicationDrlegate, so one-time initiation can go there.

Thinking about a result builder

What kind of applications would request that from a different application?