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/lib/playerModalHooks.progress.dom.test.tsx

@@ -14,7 +14,7 @@ import { renderHook } from "@testing-library/react";
 import { QueryClient } from "@tanstack/react-query";
 import { createRef, type RefObject } from "react";
 import { useYtProgress } from "./playerModalHooks";
-import { patchCachedProgress } from "./progressCache";
+import { patchCachedProgress, progressFlushes } from "./progressCache";
 import { qk } from "./queryKeys";
 import { api, type Video } from "./api";
 import type { YTPlayer } from "./youtube";
@@ -113,6 +113,27 @@ describe("useYtProgress — the checkpoint reaches the cache", () => {
     expect(cachedPos(qc)).toBe(830);
   });
 
+  it("hands its closing checkpoint out until it has landed and been patched in", async () => {
+    // A list refetched on the player's close waits for it: read before it landed, the list held the
+    // older position and, landing after the patch, put it back.
+    const qc = new QueryClient();
+    seedFeed(qc, video());
+    atPosition(830, 3118);
+    let land: (v: unknown) => void = () => {};
+    vi.spyOn(api, "saveProgress").mockReturnValue(new Promise((r) => (land = r)) as never);
+
+    const { unmount } = mountPlayer(qc, new Map());
+    await settle();
+    unmount();
+    const flushes = progressFlushes();
+    expect(flushes).toHaveLength(1);
+    land({ video_id: VID, position_seconds: 830 });
+    await flushes[0];
+    expect(cachedPos(qc)).toBe(830);
+    await settle();
+    expect(progressFlushes()).toHaveLength(0);
+  });
+
   it("mirrors what the SERVER stored, not what the player read", async () => {
     const qc = new QueryClient();
     seedFeed(qc, video({ position_seconds: 748 }));