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>

vendor/README.md

@@ -0,0 +1,20 @@
+# vendor/
+
+Third-party CSS files vendored directly into the repo so the build has no runtime npm dependency.
+
+## Contents
+
+| Directory | Package | Version | Files vendored |
+|-----------|---------|---------|----------------|
+| `open-props/` | [open-props](https://open-props.style/) | 1.7.23 | `open-props.min.css`, `media.min.css` |
+
+## How to update
+
+1. Bump the version in `package.json` and run `npm install`.
+2. Copy the new files:
+   ```bash
+   cp node_modules/open-props/open-props.min.css vendor/open-props/open-props.min.css
+   cp node_modules/open-props/media.min.css      vendor/open-props/media.min.css
+   ```
+3. Update `vendor/open-props/VERSION` with the new version and today's date.
+4. Commit all three changed files together.