fix(player): a stale session answer can no longer stop the playback that won

- The engine tracks the request out per item: A, then B, then back to A sent a second request
  for A beside the one still out.
- A hardware transcode superseded during its fallback answered with the NEWER session; the losing
  request's player then stopped it as stale. It now answers with its own (dead) session.
- A replaced hls.js instance's late manifest no longer marks the player ready or plays the
  outgoing media.
- A pause given after the answer stands: the manifest and the first canplay read the live intent.
- The debounced A/V-offset restart is dropped on an item change instead of restarting the
  previous item under the next one's title.

frontend/src/lib/plexPlayerHooks.subtitle.dom.test.tsx

@@ -402,3 +402,40 @@ describe("usePlexTracks — a <track> whose own VTT fetch failed", () => {
     expect(harness.current!.subNotice).toBeNull();
   });
 });
+
+describe("usePlexTracks — the debounced A/V-offset restart", () => {
+  afterEach(() => vi.useRealTimers());
+
+  it("is dropped when the item changes before it fires — it would restart the previous item", () => {
+    vi.useFakeTimers();
+    const restart = vi.fn();
+    const qc = client();
+    const { result, rerender } = renderHook(
+      ({ id }: { id: string }) =>
+        usePlexTracks({
+          id,
+          detail: detail(),
+          sessionK: 0,
+          subOffset: 0,
+          subPos: 88,
+          restartSession: (at) => restart(id, at),
+          setAudioTrack: () => false,
+          patchPrefs: () => {},
+          videoRef: createRef<HTMLVideoElement>(),
+          audioRef: { current: 0 } as RefObject<number | null>,
+          aoffRef: { current: 0 },
+          playhead: () => 600,
+        }),
+      {
+        wrapper: ({ children }: { children: ReactNode }) => (
+          <QueryClientProvider client={qc}>{children}</QueryClientProvider>
+        ),
+        initialProps: { id: RK },
+      },
+    );
+    act(() => result.current.changeAudioOffset(0.2));
+    rerender({ id: "next-episode" });
+    act(() => void vi.advanceTimersByTime(1000));
+    expect(restart).not.toHaveBeenCalled();
+  });
+});