Renders documents through the active theme into emit-ready pages, and
generates the synthetic pages the contract owes every site: the post
index (/blog/), one page per tag (/blog/tags/:tag/), 404.html,
and — when the site carries a portfolio — the timeline (/portfolio/)
and one story page per portfolio tag (/story/:tag/).
Raw documents keep their body verbatim; everything else renders via the three-level lookup (site overlay → theme → framework).