重构世界模拟器为模块化配方架构，完善创作编排、会话运行时与 Web UI，并清理过时技能。

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/message-branch.test.ts

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+import { describe, expect, it } from "vitest";
+import {
+  createMessageBranchState,
+  createUserVariantMessage,
+  ensureBranchForEdit,
+  recordPreMessageCheckpoint,
+  switchBranchVariant,
+} from "../src/server/message-branch.js";
+import { createSession } from "../src/runtime/phase-machine.js";
+
+describe("message-branch", () => {
+  it("switchBranchVariant restores alternate user text", () => {
+    const state = createMessageBranchState();
+    const checkpoint = {
+      runtimeSession: createSession("default"),
+      blackboardItems: [],
+    };
+    const messages = [
+      createUserVariantMessage("版本 A", "g1", 0),
+      { id: "w1", role: "system" as const, text: "回复 A", createdAt: "1" },
+    ];
+    messages[0].branchGroupId = "g1";
+    messages[0].branchIndex = 0;
+    messages[0].branchTotal = 2;
+
+    state.branches.g1 = {
+      anchorIndex: 0,
+      groupId: "g1",
+      activeIndex: 1,
+      variants: [
+        {
+          messages: [messages[0], messages[1]],
+          checkpoint,
+        },
+        {
+          messages: [
+            createUserVariantMessage("版本 B", "g1", 1),
+            { id: "w2", role: "system" as const, text: "回复 B", createdAt: "2" },
+          ],
+          checkpoint,
+        },
+      ],
+    };
+
+    const switched = switchBranchVariant(state, messages, "g1", -1);
+    expect(switched?.messages[0].text).toBe("版本 A");
+    expect(switched?.messages[1].text).toBe("回复 A");
+    expect(state.branches.g1.activeIndex).toBe(0);
+  });
+
+  it("ensureBranchForEdit captures first variant", () => {
+    const state = createMessageBranchState();
+    const messages = [
+      createUserVariantMessage("首句", "u1", 0),
+    ];
+    recordPreMessageCheckpoint(state, 0, {
+      runtimeSession: createSession("default"),
+      blackboardItems: [],
+    });
+    const branch = ensureBranchForEdit(state, messages, 0, state.preMessageCheckpoints[0]!);
+    expect(branch.variants).toHaveLength(1);
+    expect(branch.variants[0].messages[0].text).toBe("首句");
+  });
+});