Change modules from inline to nested and add lint rule to enforce use of nested modules.

[kasugaijin]

We currently use inline modules, but as things get scoped deeper, this becomse less than desirable. There are also other benefits to using nested modules: https://rubyweekly.com/link/155227/d5e73c1de1

Linter https://github.com/standardrb/standard?tab=readme-ov-file#extending-standard
Cop https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/ClassAndModuleChildren

This is not an MVP task. It can be done later.