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/registry.ts

@@ -0,0 +1,31 @@
+import type { ModuleManifest, EntityTypeRegistration, DashboardWidget } from "./module";
+
+const modules = new Map<string, ModuleManifest>();
+const entityTypes = new Map<string, EntityTypeRegistration>();
+const widgets = new Map<string, DashboardWidget>();
+
+export function registerModule(manifest: ModuleManifest): void {
+  modules.set(manifest.id, manifest);
+  for (const entity of manifest.entities) {
+    entityTypes.set(entity.type, entity);
+  }
+  for (const widget of manifest.dashboardWidgets ?? []) {
+    widgets.set(widget.id, widget);
+  }
+}
+
+export function getRegistry() {
+  return Object.freeze({
+    modules: Object.freeze([...modules.values()]),
+    entityTypes: Object.freeze([...entityTypes.values()]),
+    widgets: Object.freeze([...widgets.values()]),
+  });
+}
+
+export function getEntityType(type: string): EntityTypeRegistration | undefined {
+  return entityTypes.get(type);
+}
+
+export function getWidget(id: string): DashboardWidget | undefined {
+  return widgets.get(id);
+}