Fix runtime backend boundary enforcement

[chubes4]

Summary

• identify Codebox runtime backend packages explicitly in production-boundary enforcement
• allow declared backend packages to depend on backend-internal WordPress runtime packages
• keep the restriction intact for public/core/non-backend packages

Why

The Cloudflare runtime is a Codebox backend, but the boundary test exempted only the Playground backend. Once @automattic/wp-codebox-runtime-cloudflare added its required @wp-playground/wordpress dependency, main became red and all unrelated PRs inherited the failure.

Main failure evidence: https://github.com/Automattic/wp-codebox/actions/runs/29646139662

How to test

1. Run npm install from a fresh checkout.
2. Run npm run test:production-boundary-enforcement.
3. Run npm run build.

Compatibility

No production runtime behavior changes. This corrects the architecture test to distinguish runtime backend packages from public/core packages.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Diagnosed the pre-existing main-branch CI failure, implemented the owning-layer boundary correction, verified it, and prepared the PR under Chris Huber's direction.