feat: implement fail-fast monitoring to stop as soon as error/traceback is detected

__tests__/monitor.test.js

@@ -455,4 +455,31 @@ describe("monitorConsole", () => {
     );
     expect(api.socket.disconnect).toHaveBeenCalledTimes(1);
   });
+
+  it("exits early if a traceback occurs and onTraceback='fail'", async () => {
+    // startTick=100, monitor=10 ticks.
+    // If it didn't exit early, it would call api.time() many times.
+    const api = buildMockApi({
+      ticks: [100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110],
+    });
+
+    // Fire a traceback event after the first poll
+    setTimeout(() => {
+      api._fireConsole({
+        messages: { log: [], results: [] },
+        error:
+          "TypeError: fail-fast test\n    at Object.<anonymous> (main:1:1)",
+      });
+    }, 100);
+
+    const result = await monitorConsole(api, {
+      ...BASE_OPTS,
+      monitor: 10,
+      onTraceback: "fail",
+    });
+
+    expect(result.sawTraceback).toBe(true);
+    // We expect it to have called api.time fewer than 10 times (excluding the startTick call)
+    expect(api.time.mock.calls.length).toBeLessThan(10);
+  });
 });