add more tests

engine/lib/box.test.ts

@@ -0,0 +1,7 @@
+import { expect, test } from "vitest";
+import { type Box, box } from "./box.ts";
+
+test("box", () => {
+	const b: Box<number> = box(42);
+	expect(b).toEqual({ value: 42 });
+});