Render inline code references as HTML

[samuel-williams-shopify]

Summary

• resolve both language-prefixed inline code and legacy brace references through one HTML-node builder
• use XRB to safely construct <code><a>…</a></code> markup for resolved references
• remove recursive code-node rendering and its recursion guard
• prevent invalid nested anchors when rendering legacy references
• document the change under Unreleased

Validation

• bundle exec bake test (24 tests, 61 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib (96/96 documented)
• git diff --check