Problem cache follow-on: pluggable backends, layered cache paths, compile-options API, and offline tooling - #5117

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/

[misspell] reported by reviewdog 🐶 "unrecognised" is a misspelling of "unrecognized" Raw Output: ./src/targets/gpu/json_problem_cache.cpp:62:44: "unrecognised" is a misspelling of "unrecognized"