Build toolchains in sibling containers

[crohr]

Summary

• serve buildcurl-compatible requests from an ephemeral loopback server
• build cache misses in disposable sibling containers from the exact packaging image
• cache validated toolchain archives independently and recover corrupt immutable cache entries
• coalesce duplicate requests, cap actual compiler containers at eight, and clean them on failure or timeout
• fail closed for hosted buildcurl/barebuild domains and force the local URL after user environment values
• add direct recipe E2Es and installable Ruby, Node, and Python package fixtures

Validation

• npm test (12/12)
• npm run build and generated bundle verification
• actionlint .github/workflows/test.yml .github/workflows/scheduled-e2e.yml
• shellcheck package/e2e/install-package.sh
• Ruby, Node, and Python fixture syntax checks
• git diff --check
• GitHub Actions direct recipe and native-package application E2Es
• blank Sinatra app package, clean install, CLI execution, and PackageHall publish on EL 10, Ubuntu 26.04, and SLES 16
• cold toolchain builds, warm application-cache hits, and toolchain-cache restoration after deleting application caches

Depends on pkgr/heroku-buildpack-python#2, pkgr/heroku-buildpack-python#3, and pkgr/pkgr#190. The E2E workflows pin pkgr commit c87a5d00a3a7c4c0887161c8000cc7df8734ebed; all commit-addressed images have been published.

[crohr]

@codex review

[crohr]

CI status: both Linux unit jobs pass, including bundle reproducibility. Recipe and Ruby/Node/Python application E2Es stop at the image pull with manifest unknown because the intentionally pinned pkgr image 7a9a69d8736b90ff1af918dd1e4b84a5372df5a9 has not been published yet. No E2E reaches action or application code; rerun them after pkgr/pkgr#190 is merged and its commit-addressed images are available.

[crohr]

@codex review

[crohr]

@codex review

[crohr]

@codex review

[crohr]

Full pre-merge pipeline validation is green.

• Published all 13 pkgr target images for c87a5d00a3a7c4c0887161c8000cc7df8734ebed: https://github.com/pkgr/pkgr/actions/runs/32973635682
• Packaged and published crohr/blank-sinatra-app on EL 10, Ubuntu 26.04, and SLES 16 using the PR action and those images: https://github.com/crohr/blank-sinatra-app/actions/runs/32973962498
• Separately verified cold toolchain builds, warm application-cache hits, and toolchain-cache restores after deleting only the application caches.
• Final action-head CI passes unit tests, every direct recipe (including nested dependencies), and installable Ruby, Node, and Python native-package E2Es: https://github.com/pkgr/action/actions/runs/32975318678

Both pkgr/pkgr#190 and this PR remain open and mergeable.