refactor(core): 代码结构优化 & 添加独立 ECS 框架文档 (#316)

Core 库优化:
- Scene.ts: 提取 _runSystemPhase() 消除 update/lateUpdate 重复代码
- World.ts: 可选链简化、提取 _isSceneCleanupCandidate() 消除重复条件
- Entity.ts: 移除过度设计的 EntityComparer
- 清理方法内部冗余注释，保持代码自解释

文档改进:
- 为 core 包创建独立 README (中英文)
- 明确 ECS 框架可独立用于 Cocos/Laya 等引擎
- 添加 sparse-checkout 命令说明，支持只克隆 core 源码
- 主 README 添加 ECS 独立使用提示

README.md

@@ -29,6 +29,8 @@
 
 ---
 
+> **Just need ECS?** The core ECS framework [`@esengine/ecs-framework`](./packages/core/) can be used standalone with Cocos Creator, Laya, or any JS engine. [View ECS Documentation](./packages/core/README.md)
+
 ## Overview
 
 ESEngine is a cross-platform 2D game engine built from the ground up with modern web technologies. It provides a comprehensive toolset that enables developers to focus on creating games rather than building infrastructure.
@@ -238,13 +240,24 @@ pnpm tauri:dev
 
 ```
 esengine/
-├── packages/           # Engine packages (runtime, editor, platform)
-├── docs/               # Documentation source
-├── examples/           # Example projects
-├── scripts/            # Build utilities
-└── thirdparty/         # Third-party dependencies
+├── packages/
+│   ├── core/                    # ECS Framework (@esengine/ecs-framework)
+│   ├── math/                    # Math library (@esengine/math)
+│   ├── engine-core/             # Engine lifecycle management
+│   ├── sprite/                  # 2D sprite rendering
+│   ├── tilemap/                 # Tilemap system
+│   ├── physics-rapier2d/        # Physics engine
+│   ├── behavior-tree/           # AI behavior trees
+│   ├── editor-app/              # Desktop editor (Tauri)
+│   └── ...                      # Other modules
+├── docs/                        # Documentation source
+├── examples/                    # Example projects
+├── scripts/                     # Build utilities
+└── thirdparty/                  # Third-party dependencies
 ```
 
+> **Looking for ECS source code?** The ECS framework is in `packages/core/`
+
 ## Documentation
 
 - [Getting Started](https://esengine.cn/guide/getting-started.html)