Resume cached timelines without replaying their tail (#3329)

* perf(timeline): resume cached history from its exact range

Warm launches were repainting cached history and then downloading the same bounded tail to rediscover that nothing changed. Persist synchronization authority only when the complete current canonical window can be encoded losslessly; otherwise retain the existing display-only fallback.

* test(timeline): hide replica setup behind E2E helper

* fix(timeline): bound persisted resume overflow

* test(timeline): cover workspace eviction catch-up order

* test(timeline): read persisted range from IndexedDB

packages/app/e2e/support/helpers/timeline-pagination.ts

@@ -40,6 +40,12 @@ interface TimelineViewportSnapshot {
   scrollTop: number;
 }
 
+interface PersistedCanonicalTimelineRange {
+  epoch: string;
+  startSeq: number;
+  endSeq: number;
+}
+
 export interface TimelinePresentationSnapshot {
   marker: string;
   position: TimelinePromptPositionSnapshot;
@@ -298,6 +304,32 @@ export async function reloadAgentTimelineFromPersistedReplica(
   await expectTimelinePromptVisible(page, agent.newestPrompt);
 }
 
+export async function waitForPersistedCanonicalTimelineRange(
+  page: Page,
+  agentId: string,
+): Promise<PersistedCanonicalTimelineRange> {
+  const readRange = async () => {
+    const cache = await readReplicaCache(page);
+    if (cache?.version !== 6) return null;
+    const range = cache.hosts?.find((host) => host.timeline?.agentId === agentId)?.timeline?.range;
+    if (
+      typeof range?.epoch !== "string" ||
+      typeof range.startSeq !== "number" ||
+      typeof range.endSeq !== "number"
+    ) {
+      return null;
+    }
+    return { epoch: range.epoch, startSeq: range.startSeq, endSeq: range.endSeq };
+  };
+
+  await expect.poll(readRange).not.toBeNull();
+  const range = await readRange();
+  if (!range) {
+    throw new Error(`Persisted canonical timeline range is missing for ${agentId}`);
+  }
+  return range;
+}
+
 export async function holdNextOlderTimelinePage(
   page: Page,
   agent: LongTimelineAgent,