docs: verify Rails 7.1 ActionView template handler compatibility

[kitcommerce]

Summary

• audit Workarea for custom ActionView template handler registrations and nonstandard rendering hooks
• document the Rails 7.1 compatibility result in a focused research note
• capture the current limitation that Rails 7.1 runtime smoke testing is blocked by the existing mongoid/appraisal resolution issue

What I verified

• searched core/, admin/, and storefront/ for ActionView::Template.register_template_handler, ActionView::Template::Handlers, register_renderer, render_to_body, view_paths, lookup_context, and related rendering hooks
• confirmed Workarea does not register any custom ActionView template handlers
• confirmed the repo's existing HAML research already covers the Rails 7 handler arity requirement (call(template, source = nil))
• reviewed Workarea's Jbuilder extensions and confirmed they stay within normal JbuilderTemplate / @context.render APIs rather than custom handler internals
• noted that the rails_7_1.gemfile appraisal is currently blocked by the documented mongoid / activemodel dependency conflict, so this verification is code-audit based rather than a live Rails 7.1 smoke test

Testing

• targeted repo-wide searches for template handlers and rendering hooks
• reviewed existing HAML compatibility research and current Rails 7.1 appraisal note

Client impact

None expected.