scheduler: run duppy on a pool of domains, retire its monad, and stop rescanning the event loop - #9

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Unexpected input(s) 'opam-depext', valid inputs are ['ocaml-compiler', 'opam-repositories', 'opam-pin', 'opam-local-packages', 'opam-disable-sandboxing', 'dune-cache', 'cache-prefix', 'windows-compiler', 'windows-environment', 'allow-prerelease-opam', 'github-token']