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>

package.json

@@ -1,6 +1,10 @@
 {
   "name": "asw",
   "private": true,
+  "scripts": {
+    "build": "postcss src/main.css -o dist/asw.css",
+    "build:min": "postcss src/main.css -o dist/asw.min.css"
+  },
   "dependencies": {
     "open-props": "^1.7.0"
   },