fix: use Minitest.after_run for report summary, not at_exit

Root cause: at_exit hooks run in LIFO order. Our at_exit was registered
after minitest's autorun, so it fired before tests finished — reporter
had total=0 because no assertions had been processed yet.

Fix: Use Minitest.after_run (runs after all tests complete) when
minitest is available, falling back to at_exit for RSpec/Cucumber.

Also remove screenshots&.clear from verify_screenshots! ensure block —
clearing now happens in registry.reset after notify_reporters, so
assertions are available for the reporter before being cleared.

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

Author: pftg

lib/capybara_screenshot_diff/reporters/html.rb

@@ -136,7 +136,7 @@ def write_report
   CapybaraScreenshotDiff.reporters << CapybaraScreenshotDiff::Reporters::HTML.new(embed_images: !!ENV["CI"])
 end
 
-at_exit do
+snap_diff_finalize = proc do
   CapybaraScreenshotDiff.reporters_mutex.synchronize { CapybaraScreenshotDiff.reporters.dup }.each do |reporter|
     reporter.finalize
     if (msg = reporter.summary)
@@ -146,3 +146,9 @@ def write_report
     warn "[snap_diff] Reporter #{reporter.class} failed (#{e.class}: #{e.message})"
   end
 end
+
+if defined?(Minitest) && Minitest.respond_to?(:after_run)
+  Minitest.after_run(&snap_diff_finalize)
+else
+  at_exit(&snap_diff_finalize)
+end

lib/capybara_screenshot_diff/screenshot_assertion.rb

@@ -41,8 +41,6 @@ def self.verify_screenshots!(screenshots)
       test_screenshot_errors.compact!
 
       test_screenshot_errors.empty? ? nil : test_screenshot_errors
-    ensure
-      screenshots&.clear
     end
 
     # Asserts that an image has not changed compared to its baseline.