feat(core)!: introduce runtime extension registry and flat module conventions

BREAKING CHANGE: sup config now derives entries from package.json#exports and a server/client glob instead of manual lists; module structure follows flat + barrel convention with .server.js/.client.js runtime suffixes

src/features/admin/index.js

@@ -1,16 +1,27 @@
 /**
- * Zen Admin Module
- * Admin panel functionality with role-based access control
+ * Zen Admin — barrel serveur.
+ *
+ * - Gardes d'accès : protectAdmin, isAdmin.
+ * - Navigation : buildNavigationSections.
+ * - Registre d'extensions : registerWidget, registerWidgetFetcher, registerNavItem,
+ *   registerNavSection, registerPage (import une seule fois depuis le layout
+ *   racine de l'app consommatrice pour que les side effects s'exécutent).
+ *
+ * Client components sous @zen/core/features/admin/components.
  */
 
-// Middleware exports
-export { protectAdmin, isAdmin } from './middleware/protect.js';
-
-// Component exports (for catch-all routes)
-export { AdminPagesClient, AdminPagesLayout } from './pages.js';
-
-// NOTE: Server-only navigation builder is in '@zen/core/admin/navigation'
-// Do NOT import from this file to avoid bundling database code into client
-
-// NOTE: Admin server actions are exported separately to avoid bundling issues
-// Import them from '@zen/core/admin/actions' instead
+export { protectAdmin, isAdmin } from './protect.js';
+export { buildNavigationSections } from './navigation.js';
+export {
+  registerWidget,
+  registerWidgetFetcher,
+  registerNavItem,
+  registerNavSection,
+  registerPage,
+  collectWidgetData,
+  getWidgets,
+  getNavItems,
+  getNavSections,
+  getPage,
+  getPages,
+} from './registry.js';