fix: register framework-native hooks for RSpec and Cucumber

- Minitest: Minitest.after_run (runs after all tests)
- RSpec: RSpec.configure { |c| c.after(:suite) } (runs after suite)
- Cucumber/other: at_exit fallback

at_exit fires in LIFO order, so it runs BEFORE the test framework
finishes when registered after it. Framework-native hooks guarantee
correct ordering — reporter has data when finalize runs.

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

Author: pftg

lib/capybara_screenshot_diff/reporters/html.rb

@@ -149,6 +149,8 @@ def write_report
 
 if defined?(Minitest) && Minitest.respond_to?(:after_run)
   Minitest.after_run(&snap_diff_finalize)
+elsif defined?(RSpec)
+  RSpec.configure { |c| c.after(:suite, &snap_diff_finalize) }
 else
   at_exit(&snap_diff_finalize)
 end