Add an HTML tag to ruby2html

Tags are data: Ruby2html::Render::HTML5_TAGS / VOID_ELEMENTS in lib/gem/ruby2html/render.rb. At load, define_tag(method_name, tag, void) is called on both Ruby2html::Native (C) and Ruby2html::PureRuby. No C change is needed unless a limit is hit.

Steps

1. Add the tag as written in HTML (lowercase, hyphens allowed, e.g. turbo-frame) to HTML5_TAGS, keeping the list's ordering style. Method name = tag.tr('-', '_').
2. Void element per the HTML spec (no closing tag)? Also add it to VOID_ELEMENTS (renders <tag attrs />, ignores content and block).
3. Limits (C, ext/ruby2html/tag_render.c): name must match [a-z0-9-] and be ≤ 32 bytes (R2H_MAX_TAG_NAME); at most 200 tags (R2H_MAX_TAGS, trampoline pool). Raising a limit is a C change: update the macros/trampoline list and recompile.
4. Name clashes: the new method shadows any same-named method for code running on a Render or Ruby2html::Component (and wins over component methods/slots inside ViewComponent html blocks). Check Render/Component methods (render, plain, component, content, helpers, output, current_output, link_to, image_tag, form_with, button_to) and common Rails helpers. TemplateCompiler picks the tag up automatically (it reads HTML5_TAGS), so in .html.rb templates a receiverless call of that name will now be rewritten to the tag; search the demo app and specs for existing uses.
5. Specs in spec/gem/ inside a RenderSpecHelper.describe_each group (runs Native and PureRuby): content, attributes, block, escaping; void form if void.
6. bundle exec rake compile, CI=1 bundle exec rspec spec/gem, bundle exec rubocop lib/gem/ruby2html/render.rb.
7. Docs: mention notable new tags in llms.txt (DSL rule 1); ### Added under ## [Unreleased] in CHANGELOG.md.

Notes

• SVG/MathML children (path, circle, foreignObject) are case-sensitive and come as a family; propose that as a separate design (the 200-slot pool and [a-z0-9-] rule matter) rather than adding one tag.