/pages.md (front page /.md) serves the page's source Markdown — per-language
hybrid assembly, title injected as '# title' when the content has no h1 —
with the same ETag/Last-Modified/no-cache scheme as the HTML, which now
carries vary: accept. Pages advertise the counterpart via
<link rel=alternate type=text/markdown> and llms.txt links point at the
.md URLs.