YJIT support

[sengi]

Rails now enables YJIT for new projects by default (but it's still fine if the runtime doesn't support it).

At some point it's gonna make sense to benchmark this and see if it costs in. (It's far from a no-brainer because we're often underutilised on cores and RAM is expensive, but OTOH at peaks we do have one or two services which can run hot.)

Gonna need a Rust toolchain 🦀🦀🦀 — presumably that's easy enough on the LTS 22 base that we're using.