feat: default blur_active_element and hide_caret to true

Every real-world app enables these to prevent flaky screenshots
from blinking cursors and focused input elements. Making them
default saves boilerplate and matches universal usage pattern.

Users can still set false to opt out.

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

Author: pftg

lib/capybara_screenshot_diff.rb

@@ -27,9 +27,9 @@ module Capybara
   module Screenshot
     mattr_accessor :add_driver_path
     mattr_accessor :add_os_path
-    mattr_accessor :blur_active_element
+    mattr_accessor(:blur_active_element) { true }
     mattr_accessor :enabled
-    mattr_accessor :hide_caret
+    mattr_accessor(:hide_caret) { true }
     mattr_reader(:root) { (defined?(Rails) && defined?(Rails.root) && Rails.root) || Pathname(".").expand_path }
     mattr_accessor :stability_time_limit
     mattr_accessor :window_size