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/components/app-nav.tsx

@@ -0,0 +1,24 @@
+import Link from "next/link";
+import { getRegistry } from "@/modules/_core/registry";
+
+export function AppNav() {
+  const { modules } = getRegistry();
+  const navItems = modules.flatMap((m) => (m.nav ? [m.nav] : []));
+
+  return (
+    <nav className="border-b px-4 py-3 flex items-center gap-6">
+      <Link href="/" className="font-semibold text-sm">
+        famapp
+      </Link>
+      {navItems.map((item) => (
+        <Link
+          key={item.href}
+          href={item.href}
+          className="text-sm text-muted-foreground hover:text-foreground transition-colors"
+        >
+          {item.label}
+        </Link>
+      ))}
+    </nav>
+  );
+}