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/content/docs/_index.md

@@ -0,0 +1,11 @@
+---
+title: "Documentation"
+description: "Reference documentation for the Agentic Semantic Web framework."
+date: 2026-04-09
+tags: ["reference", "documentation"]
+
+---
+
+ASW is a single-file CSS framework for agent-generated web content. Semantic HTML is the only API. No classes, no build step, no JavaScript required.
+
+Browse the sections in the sidebar, or start with [Introduction](/docs/introduction/).