fix: keep base image when screenshots differ for reporter

Previously, base_image_path was deleted after comparison when images
differed. This meant the HTML reporter couldn't show the baseline
image — it appeared as a broken "Base" alt text.

The base image is a VCS checkout that gets re-created on next run,
so keeping it has no side effects. The reporter now shows both
baseline and current images side-by-side.

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

Author: pftg

lib/capybara_screenshot_diff/screenshot_assertion.rb

@@ -56,8 +56,6 @@ def self.assert_image_not_changed(backtrace, name, comparison)
       # Cleanup after comparisons
       if !result && comparison.base_image_path.exist?
         FileUtils.mv(comparison.base_image_path, comparison.image_path, force: true)
-      elsif !comparison.dimensions_changed?
-        FileUtils.rm_rf(comparison.base_image_path)
       end
 
       return unless result

test/unit/dsl_test.rb

@@ -152,10 +152,10 @@ def assert_no_screenshot_jobs_scheduled
       assert_not comparison.base_image_path.exist?
     end
 
-    test "#assert_image_not_changed cleans up base image when images differ" do
+    test "#assert_image_not_changed keeps base image when images differ" do
       comparison = make_comparison(:a, :b)
       assert_image_not_changed(["my_test.rb:42"], "name", comparison)
-      assert_not comparison.base_image_path.exist?
+      assert comparison.base_image_path.exist?, "base image should be kept for reporter"
     end
 
     private