fix: defer output_path computation to avoid save_path timing bug

Codex review found: auto-registered reporter captured save_path at
require time (before user configures it in test_helper.rb). Now
output_path is lazy — computed on first access, after config is set.

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

Author: pftg

lib/capybara_screenshot_diff/reporters/html.rb

@@ -9,11 +9,10 @@
 module CapybaraScreenshotDiff
   module Reporters
     class HTML
-      attr_reader :output_path, :failures, :total
+      attr_reader :failures, :total
 
       def initialize(output_path: nil, embed_images: false)
-        @output_path = Pathname.new(output_path || self.class.default_output_path)
-        @report_dir = @output_path.dirname
+        @explicit_output_path = output_path
         @embed_images = embed_images
         @failures = []
         @total = 0
@@ -44,6 +43,10 @@ def finalize
         output_path
       end
 
+      def output_path
+        @output_path ||= Pathname.new(@explicit_output_path || self.class.default_output_path)
+      end
+
       def passed = total - failures.size
       def failed = failures.size
 
@@ -88,7 +91,7 @@ def resolve_image(path)
         pathname = Pathname.new(path).expand_path
         return unless pathname.exist?
 
-        @embed_images ? data_uri(pathname) : pathname.relative_path_from(@report_dir.expand_path).to_s
+        @embed_images ? data_uri(pathname) : pathname.relative_path_from(output_path.dirname.expand_path).to_s
       end
 
       def data_uri(pathname)
@@ -98,7 +101,7 @@ def data_uri(pathname)
       end
 
       def write_report
-        FileUtils.mkdir_p(@report_dir)
+        FileUtils.mkdir_p(output_path.dirname)
         File.write(output_path, render)
       end
     end