feat(app): define trusted rewind branch flow

src/app/turnAdapter.spec.ts

@@ -1,9 +1,9 @@
 import { describe, expect, it } from "vitest";
 
 import initialView from "../../fixtures/player-view/initial.json";
-import type { PlayerView, TurnRequest } from "@contracts";
+import type { ForkBranchRequest, PlayerView, TurnRequest } from "@contracts";
 
-import { submitDemoTurn } from "./turnAdapter";
+import { forkDemoBranch, submitDemoTurn } from "./turnAdapter";
 
 function request(intent: TurnRequest["intent"], input: string): TurnRequest {
   return {
@@ -36,4 +36,28 @@ describe("local turn adapter", () => {
     expect(result.playerView.beats.some((beat) => beat.speaker === "你")).toBe(false);
     expect(result.playerView.beats.at(-1)?.speaker).toBe("娜娜");
   });
+
+  it("mirrors a fork without moving or deleting the source history", async () => {
+    const advanced = await submitDemoTurn(
+      request("speak_or_act", "我先去看看站台。"),
+      initialView as PlayerView
+    );
+    const forkRequest: ForkBranchRequest = {
+      storyId: advanced.playerView.storyId,
+      currentBranchId: advanced.playerView.branchId,
+      expectedCurrentNodeId: advanced.playerView.nodeId,
+      sourceNodeId: initialView.nodeId,
+      actionId: "fork_adapter_test"
+    };
+
+    const forked = await forkDemoBranch(forkRequest, advanced.playerView);
+
+    expect(forked.branchId).toBe("branch_fork_adapter_test");
+    expect(forked.playerView.nodeId).toBe(initialView.nodeId);
+    expect(forked.playerView.branchId).toBe(forked.branchId);
+    expect(forked.playerView.history).toHaveLength(1);
+    expect(forked.playerView.history[0]?.isCurrent).toBe(true);
+    expect(advanced.playerView.branchId).toBe(initialView.branchId);
+    expect(advanced.playerView.nodeId).toBe("node_002");
+  });
 });