Split API core tests by feature

[rtcoder]

Summary

• Keep tests/ApiCoreTest.php as a lightweight test runner.
• Move shared assertions, request helpers, and the capturing emitter into tests/Support/TestSupport.php.
• Move test-only controllers, DTOs, middleware, and auth fixtures into tests/Fixtures/TestControllers.php.
• Split API core coverage into feature files for app dispatch, middleware, modules, request/response, routing, and validation.
• Update README, REFACTORING.md, and docs/index.html to document the new test layout.

Validation

• composer validate --strict --no-check-publish
• composer dump-autoload
• find src application tests -name '*.php' -print0 | xargs -0 -n1 php -l
• composer test
• php shift.php route:list
• php shift.php health

Release

• Version label: v0.11.1