refactor: restructure repo into src/ site/ dist/ vendor/ packs/

Separate framework source from website:
- src/layers/ + src/main.css: CSS framework source (was assets/css/)
- site/: Hugo website (content/, layouts/, hugo.toml)
- dist/: built output (asw.css, asw.min.css)
- vendor/open-props/: vendored dependency with version tracking
- Hugo module mounts: dist/ → static, site runs from site/

Build: hugo --source site/ passes (105 pages).
npm run build produces dist/asw.css.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

site/layouts/partials/nav.html

@@ -0,0 +1,8 @@
+<nav>
+  <ul><li><a href="/"><strong>{{ .Site.Title }}</strong></a></li></ul>
+  <ul>
+    {{- range .Site.Menus.main }}
+    <li><a href="{{ .URL }}"{{ if $.IsMenuCurrent "main" . }} aria-current="page"{{ end }}>{{ .Name }}</a></li>
+    {{- end }}
+  </ul>
+</nav>