fix: le rejeu relit la barriere au lieu de la capturer

Dernier chemin pouvant remettre une barriere deja oubliee. Le rejeu
lisait `lastState` a la souscription et le livrait une micro-tache plus
tard ; l'etablissement peut echouer dans cet intervalle, et
`reportSetupFailure` desavoue son `State` avec lui.

La fenetre est d'exactement UN saut de micro-tache, mesuree et non
raisonnee : a zero saut l'effacement n'a pas encore eu lieu, a deux ou
plus `lastState` est deja nul donc aucun rejeu n'est mis en file et
l'arrivant ouvre une tentative neuve. `isCurrentAttempt` ne couvre pas
ce cas : cet echec n'incremente pas `attempt`, donc l'entree et la
tentative sont toutes deux encore les courantes.

DIAGNOSTIC, et cadre comme tel : `getSyncState` lit l'enregistrement et
non la poussee, donc il repond toujours `"timed-out"` et aucune lecture
ne gagne d'autorite. Ce que la livraison perimee cassait est l'attente
bootstrap de `open-repo`, qui franchissait sa barriere dessus et
journalisait `BARRIER <nuri> synced` pour une barriere qui n'existait
plus - or cette ligne de trace est celle qui distingue une absence reelle
d'une lecture non synchronisee.

J'avais ecrit ce remede puis l'avais RETIRE, faute de test qui
discrimine. L'ordonnancement ci-dessus est ce qui manquait ; le test
echoue sans le correctif et passe avec.

packages/polyfill/test/subscribe.test.ts

@@ -819,3 +819,52 @@ test("a State that lands AFTER the call was reported failed is not a barrier", a
   expect(docChannel(A).state).toBe("none");
   unsub();
 });
+
+// A JOINER IS NEVER HANDED A BARRIER THE FAN-OUT HAS ALREADY FORGOTTEN.
+//
+// The replay reads the remembered `State` when a joiner subscribes and delivers it one microtask
+// later. Read once and CAPTURED, it was delivered even when the barrier was forgotten in
+// between - and there is exactly one hop in which that happens, measured rather than reasoned:
+// the call pushes its `State`, then rejects, and a second `subscribeDoc` on the same NURI lands
+// after the rejection is dispatched and before `reportSetupFailure`'s continuation does. At zero
+// hops the clear has not happened yet; at two or more `lastState` is already null, so no replay
+// is queued at all and the joiner simply opens a fresh attempt. One hop is the whole window.
+//
+// DIAGNOSTIC, and scoped as such: `getSyncState` reads the record and not the push, so it still
+// answers `"timed-out"` and no read gains authority. What the stale delivery breaks is
+// `emulated-verifier/open-repo.ts`'s bootstrap wait, which crosses its barrier on it and logs
+// `BARRIER <nuri> synced` for a barrier that no longer exists - and that trace line is the one
+// this codebase uses to tell a genuine absence from a not-yet-synced read.
+test("a joiner is never replayed a barrier the fan-out has already forgotten", async () => {
+  let refuse: ((e: Error) => void) | null = null;
+  const doc_subscribe = mock(async (_n: string, _s: unknown, cb: (r: unknown) => void) => {
+    cb({ V0: { State: {} } }); // the barrier is remembered...
+    return await new Promise<never>((_resolve, reject) => {
+      refuse = reject; // ...while the call itself is still unanswered
+    });
+  });
+  configure({ ng: { doc_subscribe } as any, useShape: (() => {}) as any });
+  configureStoreRegistry({ getSession: async () => SESSION });
+  const first = subscribeDoc(A, () => {});
+  await tick();
+  expect(docChannel(A).barrierCrossed).toBe(true);
+
+  // The call fails, and the joiner arrives one microtask later - inside the window.
+  refuse!(new Error("RepoNotFound: the call did not take"));
+  const handedAForgottenBarrier: boolean[] = [];
+  let joiner: Unsubscribe = () => {};
+  await new Promise<void>((resolve) => {
+    queueMicrotask(() => {
+      joiner = subscribeDoc(A, () => {
+        handedAForgottenBarrier.push(!docChannel(A).barrierCrossed);
+      });
+      resolve();
+    });
+  });
+  await tick();
+
+  expect(docChannel(A).barrierCrossed).toBe(false); // the barrier is forgotten...
+  expect(handedAForgottenBarrier).toEqual([]); // ...so nobody is handed it
+  first();
+  joiner();
+});