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/single.html

@@ -0,0 +1,23 @@
+{{ define "header" }}
+  <header>
+    {{ with .Type }}<p data-text="eyebrow">{{ . }}</p>{{ end }}
+    <p data-text="dim">
+      <time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "January 2006" }}</time>
+      {{ with .Params.author }} · {{ . }}{{ end }}
+    </p>
+    {{ with .Params.tags }}
+    <p>{{ range . }}<a href="/tags/{{ . }}" data-text="tag">#{{ . }}</a> {{ end }}</p>
+    {{ end }}
+  </header>
+{{ end }}
+
+{{ define "content" }}
+  <article role="main">
+    {{ .Content }}
+    {{ with .Params.footer }}
+    <footer>
+      <small>{{ . }}</small>
+    </footer>
+    {{ end }}
+  </article>
+{{ end }}