fix(feed): an override yields only to a refetch that set out after its write was answered

Send order let a refetch read before an unanswered mark committed and drop it. A reset's position 0 is written again after a refetch that set out before the server took the reset. The player-session refetch also waits for the player's closing checkpoint, which the player now tracks (progressFlushes); refetched first, the list put the older position back.

frontend/src/components/Feed.reset.dom.test.tsx

@@ -122,6 +122,23 @@ describe("Feed — resetting a half-watched video", () => {
     await waitFor(() => expect(screen.getByText("v1@40")).toBeTruthy());
   });
 
+  it("keeps the position at 0 through a refetch that set out before the reset", async () => {
+    // That refetch read the row the reset deletes: its answer put the old position back under a
+    // card the reset had made new, and a reopen resumed there.
+    let answer: (v: unknown) => void = () => {};
+    const qc = mount(() => new Promise(() => {}));
+    await screen.findByText("v1@95");
+    vi.mocked(api.feed).mockImplementationOnce(() => new Promise((r) => (answer = r)) as never);
+    act(() => void qc.invalidateQueries({ queryKey: qk.feed() }));
+    await waitFor(() => expect(api.feed).toHaveBeenCalledTimes(2));
+    await act(() => new Promise((r) => setTimeout(r, 20)));
+    fireEvent.click(screen.getByText("reset v1"));
+    await screen.findByText("v1@0");
+    await act(async () => answer({ items: [halfWatched], next_cursor: null }));
+    await act(() => new Promise((r) => setTimeout(r, 20)));
+    expect(screen.getByText("v1@0")).toBeTruthy();
+  });
+
   it("puts the position back when the server refuses the reset", async () => {
     mount(() => Promise.reject(new Error("down")));
     fireEvent.click(await screen.findByText("reset v1"));