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- A hardware transcode superseded during its fallback answered with the NEWER session; the losing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.ts

@@ -723,6 +723,16 @@ export function usePlexTracks(opts: {
     },
     [patchPrefs, restartSession, aoffRef, playhead],
   );
+  // A restart still waiting belongs to the item it was asked on: fired after a move to the next one,
+  // it restarted the PREVIOUS item, whose stream then played under the new title. The new item's
+  // session reads the offset itself (`aoffRef`), so nothing is lost by dropping it.
+  useEffect(
+    () => () => {
+      if (aoffTimer.current) window.clearTimeout(aoffTimer.current);
+      aoffTimer.current = null;
+    },
+    [id],
+  );
 
   // --- is the selected track actually extractable yet? -------------------------------------------
   // Pulling one subtitle track out of a big MKV means ffmpeg streams the whole container: measured on