@bobstern -- I found these two articles to be straighforward and easy enough to follow / replicate / adapt:
command line
https://rderik.com/blog/creating-a-launch-agent-that-provides-an-xpc-service-on-macos/
app / GUI
https://rderik.com/blog/xpc-services-on-macos-apps-using-swift/
yes, I saw the support for some familar element types. I may look into serving in XPC via Data/Bytes (that is the format I am storing the BigInt's to disk)... but that would require a conversion to the BigInt type every time I access a value via ByteArray (even if I have already converted it 20 times prior). I would rather never do any computional 'work' that was identical more that once.
I may, as an alternative, just convert my whole app into something like a service. Currently it is just a run once command line app.
I can probably just have whole app load at start and expose it occasionaly via a GUI, command line, or services contentual menu.
Thanks, in any case... I think it might be a good time to brush up on SwiftUI and general GUI app lifecycle. I had been doing some iOS development in 2007-2009, having studied Objective-C since the MacOsX Developer Preview way back in the early 2000's.
Swift I have been picking up the last year and a half in my spare time, but have been command line focused.