perf: memoize Difference#region_area_size

Called from blank?, to_h, and tolerable? — avoid recomputing each time.

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

Author: pftg

lib/capybara/screenshot/diff/difference.rb

@@ -63,7 +63,7 @@ def blank?
         end
 
         def region_area_size
-          region&.size || 0
+          @region_area_size ||= region&.size || 0
         end
 
         def ratio