refactor: remove external_at_exit flag, rely on @Finalized guard

The external_at_exit flag was overkill — @Finalized in #finalize already
prevents double work. Simpler: at_exit always registers, framework
adapters also register native hooks, @Finalized ensures only the first
call does work.

Also move custom framework docs from README to docs/framework-setup.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

README.md

@@ -194,17 +194,6 @@ Comparisons add ~50ms per image with VIPS. Without `ruby-vips`, ChunkyPNG is use
 `DEBUG=1 bundle exec rake test` keeps `.diff.png` files for inspection.
 </details>
 
-## Custom Test Frameworks
-
-Minitest, RSpec, and Cucumber are supported out of the box. For other frameworks, call `finalize_reporters!` after all tests complete:
-
-```ruby
-# In your framework's "after suite" hook:
-CapybaraScreenshotDiff.finalize_reporters!
-```
-
-This generates the HTML report and prints the summary. Without this call, the gem falls back to `at_exit` which may fire before your tests finish.
-
 ## Installation
 
 **Requirements:** Ruby 3.2+. Rails 7.1+ for Rails integration; non-Rails projects supported via `CapybaraScreenshotDiff.serve()`. For the `:vips` driver: [libvips 8.9+](https://libvips.github.io/libvips/install.html). On macOS: `brew install vips`. On Ubuntu: `apt-get install libvips-dev`.

docs/framework-setup.md

@@ -74,4 +74,14 @@ Then('I should not see any visual difference') do
 end
 ```
 
+## Custom Test Frameworks
+
+Minitest, RSpec, and Cucumber are supported out of the box. For other frameworks, call `finalize_reporters!` in your framework's "after suite" hook:
+
+```ruby
+CapybaraScreenshotDiff.finalize_reporters!
+```
+
+This generates the HTML report and prints the summary. Without this call, the gem falls back to `at_exit` which may not fire at the right time depending on your framework's boot order.
+
 [← Back to README](../README.md)

lib/capybara_screenshot_diff/cucumber.rb

@@ -9,5 +9,4 @@
   Capybara::Screenshot::BrowserHelpers.resize_window_if_needed
 end
 
-CapybaraScreenshotDiff.external_at_exit = true
 AfterAll { CapybaraScreenshotDiff.finalize_reporters! }

lib/capybara_screenshot_diff/minitest.rb

@@ -48,7 +48,4 @@ def before_teardown
   end
 end
 
-if ::Minitest.respond_to?(:after_run)
-  CapybaraScreenshotDiff.external_at_exit = true
-  ::Minitest.after_run { CapybaraScreenshotDiff.finalize_reporters! }
-end
+::Minitest.after_run { CapybaraScreenshotDiff.finalize_reporters! } if ::Minitest.respond_to?(:after_run)

lib/capybara_screenshot_diff/reporters/html.rb

@@ -136,10 +136,6 @@ def write_report
   CapybaraScreenshotDiff.reporters << CapybaraScreenshotDiff::Reporters::HTML.new(embed_images: !!ENV["CI"])
 end
 
-# Register at_exit as fallback for frameworks without explicit adapters.
-# Framework adapters (Minitest, RSpec, Cucumber) set external_at_exit = true
-# and call finalize_reporters! via native hooks for correct ordering.
-at_exit do
-  next if CapybaraScreenshotDiff.external_at_exit?
-  CapybaraScreenshotDiff.finalize_reporters!
-end
+# Safety net for all frameworks. Framework adapters also call finalize_reporters!
+# via native hooks for correct ordering. The @finalized guard prevents double work.
+at_exit { CapybaraScreenshotDiff.finalize_reporters! }

lib/capybara_screenshot_diff/rspec.rb

@@ -42,6 +42,5 @@
     end
   end
 
-  CapybaraScreenshotDiff.external_at_exit = true
   config.after(:suite) { CapybaraScreenshotDiff.finalize_reporters! }
 end

lib/capybara_screenshot_diff/screenshot_assertion.rb

@@ -130,8 +130,6 @@ def reporters
     end
 
     attr_reader :reporters_mutex
-    attr_accessor :external_at_exit
-    alias_method :external_at_exit?, :external_at_exit
 
     def finalize_reporters!
       reporters_mutex.synchronize { reporters.dup }.each do |reporter|