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/progressCache.ts

@@ -25,6 +25,19 @@ const isItemList = (v: unknown): v is ItemList =>
  * read: the endpoint clears trivially-early and near-finished checkpoints to 0, and mirroring our
  * own reading would re-introduce that disagreement on the client.
  */
+// The player's closing flushes still on their way: its last checkpoint and the patch above. A list
+// refetched before one lands read the older checkpoint and, landing after the patch, put it back.
+const flushes = new Set<Promise<unknown>>();
+
+/** Track a closing flush until it settles (`progressFlushes` hands out the ones still running). */
+export function trackProgressFlush(flush: Promise<unknown>): void {
+  flushes.add(flush);
+  const settled = () => void flushes.delete(flush);
+  flush.then(settled, settled);
+}
+
+export const progressFlushes = (): Promise<unknown>[] => [...flushes];
+
 export function patchCachedProgress(qc: QueryClient, positions: Map<string, number>): void {
   if (positions.size === 0) return;