Use protocol HTTP middleware.

[samuel-williams-shopify]

Summary

• replace the Rack configuration with config/serve.rb and a protocol HTTP middleware application
• use sus-fixtures-protocol-http for in-process request tests
• replace Rack request environment access with request.request_path
• generate static documentation through Utopia instead of Rackula
• compose the current split redirection middleware classes
• support optional Utopia localization preferences

Dependency

This targets the unreleased Utopia 3 API currently on Utopia main. Cross-repository compatibility is validated by socketry/utopia#68, which injects its Utopia checkout into this branch.

Validation

• 11 project tests and 20 assertions pass against current Utopia
• external tests pass on Ruby 3.3, 3.4, and 4.0 across Ubuntu and macOS
• bundle exec rubocop

Direct CI remains blocked on the released Utopia 2.32 dependency until Utopia 3 is available.