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/page/single.html

@@ -0,0 +1,11 @@
+{{ define "header" }}{{ end }}
+
+{{ define "content" }}
+  <main>
+    <header>
+      <h1>{{ .Title }}</h1>
+      {{- with .Description }}<p>{{ . }}</p>{{ end }}
+    </header>
+    {{ .Content }}
+  </main>
+{{ end }}