asw-v01: drop OpenProps, native CSS tokens system

- Replace OpenProps (~25KB) with native src/tokens.css (~40 tokens)
- Remove open-props, postcss-import from package.json + postcss.config
- Update main.css: remove @import open-props/*, import ./tokens.css
- Rewrite 01-tokens.css header to reference tokens.css
- All raw OpenProps values (font stacks, sizes, weights, colors,
  easings, durations, shadows, radii, animations, media queries)
  now defined natively in tokens.css
- Build uses cat concat + cssnano (no postcss-import needed)
- Build output: 78KB minified (saved ~24KB from OpenProps removal)

src/main.css

@@ -1,18 +1,19 @@
 /* ASW — Agentic Semantic Web Framework
- * Entry point: Open Props foundation + ASW layer stack
- * PostCSS-import resolves all @imports at build time
+ * Entry point: Native CSS tokens + ASW layer stack
+ * No OpenProps. No PostCSS imports needed.
  *
- * NOTE: ALL @import statements must come first.
- * postcss-import drops any @import that follows a rule.
+ * tokens.css must come first — all layers depend on its custom properties
+ * and custom media queries.
+ *
+ * NOTE: ALL @import statements must come first in CSS.
+ * Any rule or declaration before an @import invalidates subsequent @imports.
  */
 
-/* 1. Open Props foundation */
-@import "open-props/open-props.min.css";
-@import "open-props/media.min.css";
+/* 1. Native CSS token system (replaces OpenProps foundation) */
+@import "./tokens.css";
 
 /* 2. Framework layers */
 @import "./layers/00-reset.css";
-@import "./layers/01-tokens.css";
 @import "./layers/02-typography.css";
 @import "./layers/03-landmarks.css";
 @import "./layers/04-forms.css";