Fix app stream/timeline ordering desync and init lifecycle race (#73)

* fix(app): enforce canonical stream sequencing and init resolution

* refactor: extract timeline and stream reducers to pure functions

packages/app/src/contexts/session-stream-lifecycle.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import type { AgentStreamEventPayload } from "@server/shared/messages";
+import { deriveOptimisticLifecycleStatus } from "./session-stream-lifecycle";
+
+const turnCompletedEvent: AgentStreamEventPayload = {
+  type: "turn_completed",
+  provider: "claude",
+};
+
+const turnFailedEvent: AgentStreamEventPayload = {
+  type: "turn_failed",
+  provider: "claude",
+  error: "failed",
+};
+
+describe("session stream lifecycle helpers", () => {
+  it("derives optimistic terminal lifecycle only when current status is running", () => {
+    expect(deriveOptimisticLifecycleStatus("running", turnCompletedEvent)).toBe(
+      "idle"
+    );
+    expect(deriveOptimisticLifecycleStatus("running", turnFailedEvent)).toBe(
+      "error"
+    );
+    expect(deriveOptimisticLifecycleStatus("initializing", turnCompletedEvent)).toBe(
+      null
+    );
+    expect(deriveOptimisticLifecycleStatus("idle", turnFailedEvent)).toBe(null);
+  });
+});