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/shortcodes/callout.html

@@ -0,0 +1,16 @@
+{{- /*
+  callout shortcode — wraps content in an ASW callout block.
+  
+  Usage:
+    {{< callout note >}}
+    Content here. Markdown is rendered.
+    {{< /callout >}}
+  
+  First positional param: callout type.
+  Valid types: note, warning, tip, info (maps to ASW data-callout attribute).
+  Default: note
+*/ -}}
+{{- $type := .Get 0 | default "note" -}}
+<aside data-callout="{{ $type }}">
+  {{ .Inner | markdownify }}
+</aside>