typst (PDF) and Python (HTML/Markdown) are now the only guidance engines
and the default with no flags. Delete the Gemfile, the output_format==adoc
branch (bundle exec asciidoctor[-pdf]), the adoc/ template tree, the
asciidoctor-pdf themes, and the --pdf-engine/--html-engine flags.
Canonical outputs unchanged for users: <baseline>_<lang>.pdf (typst) and
.html (Python); the intermediate is now .typ instead of .adoc. typst (the
'typst' Python package) is required for the PDF -- a missing package is a
hard error with a 'uv sync' hint (covered by a test). Keep asciidoctor.css
/ -dark.css, now inlined by the Python HTML backend. Also drop the unused
renovate config.
Render guidance HTML in pure Python (Jinja) instead of asciidoctor,
reusing the bundled asciidoctor.css inlined into a self-contained file.
The html/ template tree reuses AsciiDoctor class names so the existing CSS
styles it (cover page, numbered TOC, rules, references, admonitions).
Also harden asciidoc_to_typst to escape unbalanced */_/[/] (from regexes,
shell examples, and AsciiDoc artifacts) so every baseline compiles.
