Cover content rendering contracts.

[samuel-williams-shopify]

Summary

• exercise content markup, nodes, links, documents, responses, tags, builders, and middleware fallback behavior
• preserve nested content names during ancestor fallback resolution
• restore localized link enumeration after links storage became flat
• correct unbalanced markup diagnostics and remove an unreachable builder fallback

Verification

• bundle exec sus test/utopia/content.rb test/utopia/content/builder.rb test/utopia/content/document.rb test/utopia/content/links.rb test/utopia/content/markup.rb test/utopia/content/node.rb test/utopia/content/relative.rb test/utopia/content/response.rb test/utopia/content/tags.rb
• bundle exec rubocop $(git diff origin/main --name-only --diff-filter=ACM -- "*.rb")
• bundle exec bake decode:index:coverage lib
• git diff --check

The full local bundle exec bake test reaches generated-project tests but is blocked by an incompatible stale vendor/bundle native extension (date_core.bundle). CI runs with a clean bundle.