Defer Flipper module state initialization in Rails

[jnunemaker]

Summary

• avoid eager Flipper module-state initialization while Rails is loading
• initialize configuration and the groups registry from the Flipper engine before the default instance
• add an isolated Rails regression proving Flipper does not initialize module state before Rails defaults and preserves the required belongs-to default

This is a corrective PR stacked on #1048 so its diff contains only the lifecycle fix and regression coverage.

Testing

• bundle exec rspec spec/flipper/engine_spec.rb spec/flipper_spec.rb spec/flipper/cloud_spec.rb
• 130 examples, 0 failures
• git diff --check