Handle cross-cutting concerns in a better way

[grunt-lucas]

Logging, diagnostics, etc.

We should create interfaces for diagnostics, logging, etc. DiagEngine can become an interface, and the current DiagEngine class should become DiagEngineImpl or some such. That way, when testing, we can inject mocks/stubs more easily.

https://gameprogrammingpatterns.com/service-locator.html
https://johnfarrier.com/the-service-locator-pattern-a-robust-c-implementation
https://blog.ploeh.dk/2024/09/02/keeping-cross-cutting-concerns-out-of-application-code
https://jessemcdowell.ca/2024/05/Cross-Cutting-Concerns
https://softwareengineering.stackexchange.com/questions/318395/are-so-called-cross-cutting-concerns-a-valid-excuse-to-break-solid-di-ioc