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/browser/agent-timeline-pagination.spec.ts

@@ -26,9 +26,41 @@ import {
   expectTimelineViewportAnchoredAfterPrepend,
   userNavigatesTimelineToHistoryStartWithKeyboard,
   userScrollsTimelineToHistoryStart,
+  waitForPersistedCanonicalTimelineRange,
 } from "../support/helpers/timeline-pagination";
+import { trackAgentTimelineRequests } from "../support/helpers/agent-timeline-gate";
+import { seedMockAgentWorkspace } from "../support/helpers/mock-agent";
 
 test.describe("Agent timeline pagination", () => {
+  test("resumes a persisted canonical timeline after its exact end", async ({ page }) => {
+    const prompt = "timeline persisted range resumes without tail replay";
+    const agent = await seedMockAgentWorkspace({
+      repoPrefix: "timeline-persisted-range-",
+      title: "Persisted timeline range",
+      initialPrompt: prompt,
+    });
+    try {
+      await agent.client.waitForFinish(agent.agentId, 15_000);
+      await openAgentTimeline(page, agent);
+      await expectTimelinePromptVisible(page, prompt);
+      const range = await waitForPersistedCanonicalTimelineRange(page, agent.agentId);
+
+      const tracker = await trackAgentTimelineRequests(page, agent.agentId);
+      await page.reload();
+      await expectTimelinePromptVisible(page, prompt);
+
+      const request = await tracker.nextRequest();
+      expect(request).toEqual({
+        direction: "after",
+        cursor: { epoch: range.epoch, seq: range.endSeq },
+      });
+      await tracker.waitForResponse();
+      expect(tracker.requests().some((entry) => entry.direction === "tail")).toBe(false);
+    } finally {
+      await agent.cleanup();
+    }
+  });
+
   test("keeps a fixed history-start gutter before, during, and after pagination", async ({
     page,
   }) => {