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/meta/seo.html

@@ -0,0 +1,16 @@
+{{- /*
+  partials/meta/seo.html — canonical URL + robots directives
+
+  Outputs:
+    <link rel="canonical">      Always emitted; uses .Permalink (absolute).
+    <meta name="robots">        "noindex, nofollow" on draft/future pages,
+                                "index, follow" otherwise.
+
+  No params required. Works from any page context.
+*/ -}}
+<link rel="canonical" href="{{ .Permalink }}">
+{{- if or .Draft (gt .Date now) }}
+<meta name="robots" content="noindex, nofollow">
+{{- else }}
+<meta name="robots" content="index, follow">
+{{- end }}