refactor: change loadModel parameter from string to string array

- Update loadModel signature: url: string -> urls: string[]
- Update validation logic for array parameter
- Update all demo files to pass array instead of single string

src/managers/engine-manager.ts

@@ -234,15 +234,15 @@ export class EngineManager extends BaseManager {
 
     /**
      * 加载模型
-     * @param url 模型 URL
+     * @param urls 模型 URL 数组
      * @param options 加载选项
      */
-    public loadModel(url: string, options?: ModelLoadOptions): void {
+    public loadModel(urls: string[], options?: ModelLoadOptions): void {
         if (!this.engineInstance) {
             console.warn('[EngineManager] 3D Engine not initialized.');
             return;
         }
-        this.engineInstance.loadModel(url, options);
+        this.engineInstance.loadModel(urls, options);
     }
 
     /**