feat(modules): add external module system with auto-discovery and public pages support

- add `src/core/modules/` with registry, discovery (server), and public index
- add `src/core/public-pages/` with registry, server component, and public index
- add `src/core/users/permissions-registry.js` for runtime permission registration
- expose `./modules`, `./public-pages`, and `./public-pages/server` package exports
- rename `registerFeatureRoutes` to `registerApiRoutes` with backward-compatible alias
- extend `seedDefaultRolesAndPermissions` to include module-registered permissions
- update `initializeZen` and shared init to wire module discovery and registration
- add `docs/MODULES.md` documenting the `@zen/module-*` authoring contract
- update `docs/DEV.md` with references to module system docs

package.json

@@ -93,6 +93,15 @@
     "./users/constants": {
       "import": "./dist/core/users/constants.js"
     },
+    "./modules": {
+      "import": "./dist/core/modules/index.js"
+    },
+    "./public-pages": {
+      "import": "./dist/core/public-pages/index.js"
+    },
+    "./public-pages/server": {
+      "import": "./dist/core/public-pages/PublicModulePage.server.js"
+    },
     "./api": {
       "import": "./dist/core/api/index.js"
     },