feat(fp): WP-14 — Storybook taxonomy reorg + Layers MDX

Retitle all 49 stories into a sidebar that makes the DDD seam visible:
Foundations (curriculum) -> Design System (Atoms/Molecules/Organisms/
Templates/Devtools, everything in shared/ui + shared/layout) -> Domein
(Registratie/Herregistratie/Auth/Brief, everything in a context's ui/).
Pin the order via storySort. Add layers.mdx explaining the split and
linking the enforcing eslint rules; document the story-title convention
in CLAUDE.md. Fix a stale "status banner" reference in atomic-design.mdx
left over from WP-13's upload-status-banner deletion.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.storybook/preview.ts

@@ -25,6 +25,18 @@ const preview: Preview = {
         date: /Date$/i,
       },
     },
+    // Sidebar tells the DDD seam: reusable design system, then domain contexts.
+    // See src/docs/layers.mdx.
+    options: {
+      storySort: {
+        order: [
+          'Foundations',
+          'Design System',
+          ['Atoms', 'Molecules', 'Organisms', 'Templates', 'Devtools'],
+          'Domein',
+        ],
+      },
+    },
   },
 };