Extend best-practice list: avoid using static "global mockable state" providers/top-level functions in the favour of injectable ones

It would be nice to recommend avoiding using "global" state in the libraries, that later is almost impossible to test and/or mock.

We tend to favor explicitly spelled-out APIs for the such state: datetime's `Clock.System.now()`, Okio's `FileSystem.DEFAULT.read*` as opposed to top-level `now()`, `read(path)` etc. (the most notable existing API that falls into this trap is coroutine's `Dispatchers.Default`)

Start by locating the best-practice list in the repository and read the surrounding guidance for library API design. Use the issue's Clock.System.now(), FileSystem.DEFAULT.read*, and Dispatchers.Default examples to frame the recommendation; done means the guideline clearly explains preferring injectable, explicitly scoped state over top-level or global providers.

Written by the indexing model from the issue text.