feat(11.1-02): add the documentation theme, chroma highlighting and docs:serve

- WinterCMS-style shell: header with search and theme toggle, grouped
  sidebar, on-page TOC, pager, page actions, callouts, heading
  permalinks, footer and a 404 page
- fenced code highlighted at build time by chroma/v2 into tok-* classes,
  with a copy button; no inline script, style or handler
- vendored DM Sans/DM Mono fonts and Lucide icons with their licences
- client-side search over search-index.json built with textContent only
- summer docs:serve builds into a temp dir, serves on loopback by default,
  returns 404.html with status 404 and rebuilds on change

README.md

@@ -148,6 +148,8 @@ npm --prefix admin run gen:api     # regenerate TypeScript types from admin/open
 
 `summer docs:build` renders `docs/` and every module README into a static site under `site/` (use `--out` for another directory, `--check` to validate without writing). Code blocks with a `src=` reference are copies of real source; after changing that source, run `summer docs:sync` to refresh the copies.
 
+`summer docs:serve` builds the same site into a temporary directory and previews it at `http://127.0.0.1:8088` (`--addr` to change it), rebuilding when the docs, a module or a `src=` source changes; a failed rebuild prints its problems and keeps serving the last good build. It listens only on a loopback address unless you pass `--allow-remote`. Search needs this server: browsers block the search index over `file://`.
+
 ## Design notes
 
 Decisions and background live in [`.planning/notes/`](.planning/notes/), including: