single stable boundary

[codefather2026]

Closes #384

Summary

• add a focused mock API facade between the public API barrel and the large mock implementation
• route all application components, including the developer scenario selector and navigation, through @/lib/api
• keep implementation-only fault injection controls out of the application API surface
• add a boundary contract test for the mock controls used by application code

Testing

• Not run locally: dependencies are not installed and this repository does not contain a lockfile; dependency installation did not complete in the execution environment.
• Source validation: git diff --check
• Boundary audit: no application component or page imports lib/api/mock directly