[SandObserver]

Summary

• Cache the Debian packages that playwright install --with-deps downloads, keyed by runner image and package-lock.json.
• On a cache hit, install them with dpkg -i and install only the browsers. When the cached set does not fit the runner image, repair with apt-get -f install and fall back to the full --with-deps install.
• Keep downloaded packages during the full install so the cache can be saved.

Testing
All green. The first e2e run filled the cache; the next installed from it in 17 s.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.