design for runtime configuration

[d-v-b]

I think the way we handle configuration for zarr-python could be vastly improved. I had claude write up a summary document, and a plan, here: https://gist.github.com/d-v-b/c5468b23e7eb127d14de99807f4be7f5. The tl;dr is that there are many opportunities to move our configuration from "magic mutable global object" to "plain data". We already have some work started, e.g. #4101.

I'm opening this discussion in case anyone wants to chat about the broader project of improving our config.