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/og.html

@@ -0,0 +1,41 @@
+{{- /*
+  partials/meta/og.html — Open Graph + Twitter Card meta tags
+
+  og:title        Page title (site title for home page)
+  og:description  .Description → .Site.Params.description → ""
+  og:url          .Permalink
+  og:type         "website" (home) | "article" (everything else)
+  og:site_name    .Site.Title
+  og:image        .Params.image → .Site.Params.og_image → absent
+  twitter:card    "summary_large_image" when image present, else "summary"
+  twitter:site    .Site.Params.twitter (optional @handle)
+
+  Configure in hugo.toml:
+    [params]
+      og_image = "/images/og-default.png"   # fallback OG image
+      twitter   = "@yourhandle"             # omit if not on Twitter/X
+*/ -}}
+{{- $title := cond .IsHome .Site.Title (printf "%s · %s" .Title .Site.Title) -}}
+{{- $desc  := or .Description .Site.Params.description "" -}}
+{{- $image := or .Params.image .Site.Params.og_image "" -}}
+{{- $type  := cond .IsHome "website" "article" -}}
+
+<meta property="og:title"       content="{{ $title }}">
+<meta property="og:description" content="{{ $desc }}">
+<meta property="og:url"         content="{{ .Permalink }}">
+<meta property="og:type"        content="{{ $type }}">
+<meta property="og:site_name"   content="{{ .Site.Title }}">
+{{- with $image }}
+<meta property="og:image"       content="{{ . | absURL }}">
+{{- end }}
+
+{{- /* Twitter / X Card */}}
+<meta name="twitter:card"        content="{{ if $image }}summary_large_image{{ else }}summary{{ end }}">
+<meta name="twitter:title"       content="{{ $title }}">
+<meta name="twitter:description" content="{{ $desc }}">
+{{- with $image }}
+<meta name="twitter:image"       content="{{ . | absURL }}">
+{{- end }}
+{{- with .Site.Params.twitter }}
+<meta name="twitter:site"        content="{{ . }}">
+{{- end }}