- 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
