fix: preserve history coverage during tail refresh

Keeps an existing complete cursor/coverage after refreshing the latest messages
Prevents tail refreshes from reintroducing an incorrect "load older" state
Adds a regression test for complete history coverage across refreshes

packages/ui/src/sync/session-message-loader.test.ts

@@ -89,6 +89,28 @@ describe("SessionMessageLoader", () => {
     childStores.disposeAll()
   })
 
+  test("preserves complete history coverage across a tail refresh", async () => {
+    let calls = 0
+    const { childStores, loader } = createLoader(async ({ sessionID }) => {
+      calls += 1
+      return calls === 1
+        ? response([createRecord(sessionID, "msg_1")])
+        : response([createRecord(sessionID, "msg_2")], "stale-tail-cursor")
+    })
+    const target = { directory: "/repo", sessionID: "session-a" }
+
+    await loader.ensure(target)
+    expect(loader.getSnapshot(target).complete).toBe(true)
+    expect(loader.getSnapshot(target).cursor).toBe(undefined)
+
+    await loader.refreshTail(target, 2)
+
+    expect(loader.getSnapshot(target).complete).toBe(true)
+    expect(loader.getSnapshot(target).cursor).toBe(undefined)
+    loader.dispose()
+    childStores.disposeAll()
+  })
+
   test("does not deduplicate identical session IDs across directories", async () => {
     const calls: string[] = []
     const { childStores, loader } = createLoader(async ({ directory, sessionID }) => {