Organize framework namespaces by domain

[rtcoder]

Summary

• Move response classes into the Engine\Response namespace and directory.
• Move router classes into the Engine\Routing\Router namespace and directory.
• Move service container contracts into the Engine\Service namespace and directory.
• Update app, module, CLI, tests, and README references to the new namespace layout.
• Fix route type imports after the router namespace refactor.
• Make the API workflow read live PR labels when validating the release version label.
• Mark completed roadmap items and next refactoring steps in REFACTORING.md.

Validation

• composer dump-autoload
• find Engine application tests -name '*.php' -print0 | xargs -0 -n1 php -l
• composer test
• php shift.php route:list
• php shift.php health
• HTTP smoke tests:
  • GET /health -> 200 application/json
  • GET /hello -> 404 application/json

Release

• Version label: v0.8.1