test(player): pin the stop URL's ids, the beacon's settle, the kept point's edges

The stop-URL contract passed with the two ids swapped and read the backend
from the working directory; nothing called a real beacon's onSettled or its
library pass-through; the kept-point rule was held by its constants only.

frontend/src/lib/api/core.test.ts

@@ -454,3 +454,31 @@ describe("uploadDownload's wire shape", () => {
     expect(FakeXHR.last.headers["X-Upload-Display-Name"]).toBeUndefined();
   });
 });
+
+// The library player re-reads its listings once its closing point is on the server: it cancelled
+// their poll so an older answer could not put the point back, and only this callback brings them
+// back. A beacon that never called it, or a slice that dropped it, left the lists stale with every
+// test green.
+describe("a beacon's settle callback", () => {
+  it("runs once the request is answered, and once it has failed", async () => {
+    const fetch = vi
+      .spyOn(globalThis, "fetch")
+      .mockResolvedValueOnce(new Response(null, { status: 204 }))
+      .mockRejectedValueOnce(new TypeError("offline"));
+    const answered = vi.fn();
+    const failed = vi.fn();
+    beacon("/api/a", {}, answered);
+    beacon("/api/b", {}, failed);
+    await vi.waitFor(() =>
+      expect([answered.mock.calls.length, failed.mock.calls.length]).toEqual([1, 1]),
+    );
+    expect(fetch.mock.calls.map(([url]) => url).slice(-2)).toEqual(["/api/a", "/api/b"]);
+  });
+
+  it("is passed on by the library progress beacon", async () => {
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(new Response(null, { status: 204 }));
+    const settled = vi.fn();
+    downloadsApi.saveDownloadProgressBeacon(7, 125.4, 600, settled);
+    await vi.waitFor(() => expect(settled).toHaveBeenCalledTimes(1));
+  });
+});