Single source in-development fork name

`tox.ini` currently contains a hardcoded reference to Osaka. This'll be a problem when switching to developing the next fork.

We should single source this information, perhaps using the `Hardfork` class.

I'm thinking something like `$ ethereum-spec-forks -n N --test-name` (or whatever) where `N` is the fork number (0 being Frontier and `-1` being the most recent).
_Originally posted by @SamWilsn in https://github.com/ethereum/execution-specs/pull/1280#discussion_r2213648976_