Extract Rails integration from lib/comma.rb

[eitoball]

Summary

Move Rails-specific integration (lazy hooks and the render csv: renderer) out of lib/comma.rb into dedicated files under lib/comma/rails/.

Problem

lib/comma.rb currently mixes concerns:

• Gem boot and requires
• ActiveSupport.on_load hooks for ActiveRecord, Mongoid, ActionController
• ~30 lines registering the :csv renderer (filename, BOM, MIME type, option slicing, send_data)

This makes the entry point harder to read and means Rails integration cannot be tested or reviewed in isolation without scrolling through unrelated code.

Proposed approach

lib/comma.rb                    # thin loader only
lib/comma/rails.rb              # on_load hooks, require integration files
lib/comma/rails/renderer.rb     # ActionController::Renderers.add :csv

Keep the same lazy-loading behavior via ActiveSupport.on_load(:action_controller).

Optional follow-up (separate issue): require comma/rails only when Rails is present; today the hooks are in the main require path but no-op until frameworks load.

Files likely involved

• lib/comma.rb (slim down)
• New: lib/comma/rails.rb, lib/comma/rails/renderer.rb
• spec/controllers/users_controller_spec.rb (integration regression)
• spec/spec_helper.rb (load path unchanged for tests)

Acceptance criteria

• require 'comma' behavior unchanged for non-Rails apps
• render csv: User.all still works in controller specs
• All renderer options (filename, extension, BOM, write_headers, etc.) still pass controller specs
• lib/comma.rb no longer contains renderer implementation details
• No public API change

Labels (suggested)

refactor

Depends on

Optional: #3 (options normalization) — renderer extraction is simpler if options parsing already lives in Comma::Options.