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/head.html

@@ -0,0 +1,41 @@
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="color-scheme" content="dark light">
+<title>
+  {{- if .IsHome -}}
+    {{ .Site.Title }}
+  {{- else -}}
+    {{ .Title }} · {{ .Site.Title }}
+  {{- end -}}
+</title>
+{{- with .Description }}<meta name="description" content="{{ . }}">{{- end }}
+{{- if not .Description }}{{- with .Site.Params.description }}<meta name="description" content="{{ . }}">{{- end }}{{- end }}
+
+{{- /* ── Meta partials ─────────────────────────────────────────── */}}
+{{- partial "meta/seo.html"           . -}}
+{{- partial "meta/og.html"            . -}}
+{{- partial "meta/ai-disclosure.html" . -}}
+{{- partial "meta/json-ld.html"       . -}}
+
+{{- /* ── CSS ────────────────────────────────────────────────────── */}}
+{{- if hugo.IsDevelopment }}
+  <link rel="stylesheet" href="/vendor/open-props.min.css">
+  <link rel="stylesheet" href="/vendor/media.min.css">
+  <link rel="stylesheet" href="/css/layers/00-reset.css">
+  <link rel="stylesheet" href="/css/layers/01-asw.css">
+  <link rel="stylesheet" href="/css/layers/02-semantic.css">
+  <link rel="stylesheet" href="/css/layers/03-components.css">
+  <link rel="stylesheet" href="/css/layers/04-data-attrs.css">
+  <link rel="stylesheet" href="/css/layers/05-utilities.css">
+  <link rel="stylesheet" href="/css/layers/06-charts.css">
+  <link rel="stylesheet" href="/css/layers/07-chroma.css">
+  <link rel="stylesheet" href="/css/layers/08-layout.css">
+  <link rel="stylesheet" href="/css/layers/08a-paper.css">
+  <link rel="stylesheet" href="/css/layers/09-landing.css">
+{{- else }}
+  <link rel="stylesheet" href="/asw.css">
+{{- end }}
+
+{{- range .AlternativeOutputFormats -}}
+<link rel="{{ .Rel }}" type="{{ .MediaType.Type }}" href="{{ .Permalink | safeURL }}">
+{{- end }}