feat(core): 添加持久化实体支持跨场景迁移

实现实体生命周期策略，允许标记实体为持久化，在场景切换时自动迁移到新场景。

主要变更：
- 新增 EEntityLifecyclePolicy 枚举（SceneLocal/Persistent）
- Entity 添加 setPersistent()、setSceneLocal()、isPersistent API
- Scene 添加 findPersistentEntities()、extractPersistentEntities()、receiveMigratedEntities()
- SceneManager.setScene() 自动处理持久化实体迁移
- 添加完整的中英文文档和 21 个测试用例

docs/.vitepress/config.mjs

@@ -45,7 +45,8 @@ function createSidebar(t, prefix = '') {
             link: `${prefix}/guide/scene`,
             items: [
               { text: t.sidebar.sceneManager, link: `${prefix}/guide/scene-manager` },
-              { text: t.sidebar.worldManager, link: `${prefix}/guide/world-manager` }
+              { text: t.sidebar.worldManager, link: `${prefix}/guide/world-manager` },
+              { text: t.sidebar.persistentEntity, link: `${prefix}/guide/persistent-entity` }
             ]
           },
           {

docs/.vitepress/i18n/en.json

@@ -23,6 +23,7 @@
     "scene": "Scene",
     "sceneManager": "SceneManager",
     "worldManager": "WorldManager",
+    "persistentEntity": "Persistent Entity",
     "behaviorTree": "Behavior Tree",
     "btGettingStarted": "Getting Started",
     "btCoreConcepts": "Core Concepts",

docs/.vitepress/i18n/zh.json

@@ -23,6 +23,7 @@
     "scene": "场景管理 (Scene)",
     "sceneManager": "SceneManager",
     "worldManager": "WorldManager",
+    "persistentEntity": "持久化实体 (Persistent Entity)",
     "behaviorTree": "行为树系统 (Behavior Tree)",
     "btGettingStarted": "快速开始",
     "btCoreConcepts": "核心概念",