refactor: avoid double finalize call for Minitest

Restore if/else — Minitest.after_run when Minitest is loaded, at_exit
otherwise. Nobody runs Minitest and RSpec in the same process, so the
@Finalized double-execution guard is unnecessary overhead.

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

Author: pftg

lib/capybara_screenshot_diff/reporters/html.rb

@@ -147,8 +147,8 @@ def write_report
   end
 end
 
-# Always register at_exit for RSpec/Cucumber/other frameworks.
-# Additionally register Minitest.after_run for correct ordering with Minitest.
-# The @finalized guard in #finalize prevents double execution.
-at_exit(&snap_diff_finalize)
-Minitest.after_run(&snap_diff_finalize) if defined?(Minitest) && Minitest.respond_to?(:after_run)
+if defined?(Minitest) && Minitest.respond_to?(:after_run)
+  Minitest.after_run(&snap_diff_finalize)
+else
+  at_exit(&snap_diff_finalize)
+end