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/_default/_markup/render-footnotes.html

@@ -0,0 +1,13 @@
+{{- /* render-footnotes.html
+     Replace Hugo's default <div class="footnotes"> with ASW-semantic markup.
+     Hugo v0.123.0+ render hook for the footnote block.
+     Ref: https://gohugo.io/render-hooks/footnotes/
+*/ -}}
+<footer data-role="footnotes">
+  <hr>
+  <ol>
+    {{- range .Items }}
+    <li id="{{ .ID }}">{{ .Content }}{{ .Return }}</li>
+    {{- end }}
+  </ol>
+</footer>