Pre-initialize global rayon threadpool (#41226)

We only use it a handful of times and the default amount of threads
(logical cpu core number) its spawns is overkill for this. This also
gives the threads names oppose to being labeled `<unknown>`

Release Notes:

- N/A *or* Added/Fixed/Improved ...
2471ae451cLukas Wirth committed on 10/26/2025, 12:44:34 PM· committed by GitHubparentd6b31d8
8 files changedLine totals unavailable