docs/refactor: rename getModuleMetadata and update route auth format

- Rename `getModuleMetadata` to `getModuleMetadataGenerator` in registry,
  index, and client exports to clarify its purpose (returns a generator
  function, not a metadata object)
- Add new `getModuleMetadata` and `getMetadataGenerator` exports from
  `modules.metadata.js` for server-side metadata object retrieval
- Update route auth format in docs from `requireAuth`/`requireAdmin`
  flags to a single `auth` field with values: `'admin'`, `'user'`,
  or `'public'`
- Fix `isModuleEnabledInEnv` to replace hyphens with underscores in
  env var names (e.g. `my-module` → `ZEN_MODULE_MY_MODULE`)
- Replace `useState` initializer in `ZenProvider` with `useRef` guard
  to avoid React strict mode double-invocation issues

src/features/provider/ZenProvider.js

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState } from 'react';
+import { useEffect, useRef } from 'react';
 import { ToastProvider, ToastContainer } from '@zen/core/toast';
 import { registerExternalModulePages } from '../../modules/modules.pages.js';
 
@@ -15,13 +15,16 @@ import { registerExternalModulePages } from '../../modules/modules.pages.js';
  * @param {ReactNode} props.children
  */
 export function ZenProvider({ modules = [], children }) {
-  // Register external module pages once, synchronously, before first render.
-  // useState initializer runs exactly once and does not cause a re-render.
-  useState(() => {
+  const registered = useRef(false);
+
+  if (!registered.current) {
+    // Register synchronously on first render so pages are available
+    // before any child component resolves a module route.
     if (modules.length > 0) {
       registerExternalModulePages(modules);
     }
-  });
+    registered.current = true;
+  }
 
   return (
     <ToastProvider>