Collocate types and logic by removing `types` and `runtime` directories

It's not the [first time I have questioned whether a type should be in a runtime module or types](https://github.com/canonical/superhref/pull/4#discussion_r3541486269). How do you feel about dropping the runtime/types split altogether and instead splitting the modules by their responsibility only: logic and types coexisting together?

_Originally posted by @steciuk in https://github.com/canonical/superhref/pull/12#discussion_r3552331895_

Start by reviewing the existing `types` and `runtime` directories and the linked pull-request discussions, then map which modules contain types versus logic. The work is done when the runtime/types split is removed and modules are organized only by responsibility, with types and logic coexisting where appropriate.

Written by the indexing model from the issue text.