Run independent CI steps in parallel

[caugner]

GitHub Actions can now run steps in parallel via parallel step groups (or background: true + wait-all): https://github.blog/changelog/2026-06-25-actions-steps-can-now-be-run-in-parallel/

Independent, network-bound setup steps (repository checkouts, artifact downloads, toolchain setup) can overlap instead of running serially.

• mdn/rari: ci: run independent steps in parallel rari#775
• mdn/fred: _build.yml, test.yml, npm-publish-simulation.yml
• mdn/content: ci(pr-review-companion): run independent steps in parallel content#45580
• mdn/translated-content: ci: run independent steps in parallel translated-content#38365