refactor: clean up file size check variables in quick_equal?

Extract base_size/new_size for clarity. Prepares for future
file size ratio pre-rejection.

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

Author: pftg

lib/capybara/screenshot/diff/image_compare.rb

@@ -61,10 +61,11 @@ def initialize(image_path, base_image_path, options = {})
         #
         # @note This method will raise ArgumentError if either image file is missing.
         def quick_equal?
-          # Quick file size check - if sizes are equal, perform a simple file comparison
-          if base_image_path.size == image_path.size
-            # If we have identical files (same size and content), we can return true immediately
-            # without more expensive comparison
+          base_size = base_image_path.size
+          new_size = image_path.size
+
+          # Exact same file size — likely identical, verify with byte comparison
+          if base_size == new_size
             return true if files_identical?(base_image_path, image_path)
           end