Refactor: v3 loader/autoloader, remove legacy, add tests

Summary:
- Implement v3 autoloader and update `load_lib()` to rely on class autoloading.
- Improve autoloader filename mapping (kebab-case, legacy support).
- Move core controllers to `includes/controller/` and remove `includes/legacy/` and `t/` directories (archived in SVN).
- Fix dry-run trace/log newline handling (`trace()` and `log_msg()`).
- Add PHPUnit tests: php-lint smoke test, reflection-based methods test, dry-run/log newline tests, and loader singleton tests.

Notes:
- This initial commit prepares the plugin for Git; run the lint and tests locally before publishing.