feat(mockups): slot 13 curtain call; allow three.js in 13

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/server.test.ts

@@ -8,7 +8,7 @@ let base: string;
 beforeAll(() => { server = startServer(0); base = `http://localhost:${server.port}`; });
 afterAll(() => server.stop(true));
 
-test("registry has 12 mockups", () => expect(mockups).toHaveLength(12));
+test("registry has 13 mockups", () => expect(mockups).toHaveLength(13));
 
 test("index links every mockup", async () => {
   const res = await fetch(base + "/");