add spec that eager loads the application

spec/integration/zeitwerk_compliance_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe "Zeitwerk compliance" do
+  it "eager loads all files without errors" do
+    expect { Rails.application.eager_load! }.not_to raise_error
+  end
+end