Add Rails engine with browser UI at /whetrails

Mounts a Rails engine that lists app models in a sidebar and renders
the full callback/validator chain for any model. Source locations are
linked via vscode:// URIs. Condition badges and concern module tags
distinguish at-a-glance where each callback comes from.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/whetrails.rb

@@ -1,6 +1,7 @@
 require_relative "whetrails/version"
 require_relative "whetrails/chain"
 require_relative "whetrails/inspector"
+require_relative "whetrails/engine" if defined?(Rails)
 
 module Whetrails
   def self.inspect_model(model_class)

lib/whetrails/engine.rb

@@ -0,0 +1,9 @@
+module Whetrails
+  class Engine < ::Rails::Engine
+    isolate_namespace Whetrails
+
+    # In the gem's dev setup, config/routes.rb belongs to the host app.
+    # Point the engine's routing path to a dedicated file.
+    config.paths.add "config/routes.rb", with: "lib/whetrails/engine_routes.rb"
+  end
+end

lib/whetrails/engine_routes.rb

@@ -0,0 +1,4 @@
+Whetrails::Engine.routes.draw do
+  resources :models, only: [:index, :show]
+  root to: "models#index"
+end