Add module loader & registry (task 04)

Implements the extensibility foundation: ModuleManifest/DashboardWidget/EntityTypeRegistration types in _core/module.ts, a plain-Map registry singleton with registerModule/getRegistry/getEntityType/getWidget, and stub manifests for calendar/lists/notes. Root layout imports src/modules/index.ts for side-effect registration; AppNav reads the registry to render nav links. /debug/registry dumps the full registry JSON in dev using zod v4 + z.toJSONSchema(). Adding a fourth module requires one folder + one line in index.ts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/_core/index.ts

@@ -0,0 +1,12 @@
+export type {
+  ModuleManifest,
+  EntityTypeRegistration,
+  DashboardWidget,
+  QuickAddAction,
+  WidgetContext,
+  ShareCapabilities,
+  ReminderCapabilities,
+  SearchAdapter,
+  SearchResult,
+} from "./module";
+export { registerModule, getRegistry, getEntityType, getWidget } from "./registry";