test: establish frontend gate taxonomy

tests/unit/system-clock.test.js

@@ -0,0 +1,13 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { systemClock } from "../../src/application/ports/clock-port.js";
+
+describe("systemClock", () => {
+  it("resolves after the requested duration", async () => {
+    vi.useFakeTimers();
+    const sleeper = systemClock.sleep(250);
+    await vi.advanceTimersByTimeAsync(250);
+    await expect(sleeper).resolves.toBeUndefined();
+    vi.useRealTimers();
+  });
+});