Bun: cgroup-aware AvailableParallelism / HardwareConcurrency on Linux

Something from the PR description that I had to look up to understand: "WTF" is "WebKit Template Framework".

I feel like https://github.com/oven-sh/WebKit/commit/35315978baee84ed1bd... is the more interesting commit tbh

I had high hopes for Bun, but looks like it has gone down the shitter after they went all in on vibecoding.