Cherry-pick PR #2271's config refactor onto release/2.0-rc.
Used 'git merge -X theirs' to auto-resolve content conflicts in favor of
the PR's design (frozen AppConfig + explicit-parameter passing).
Limitations:
- Release-only changes that overlapped with PR's refactor in 119 files
are NOT preserved — those files reflect PR's version. Follow-up commits
on this branch will need to re-apply release-only modifications where
meaningful.
- See PR #2271 for design rationale.