feat(demo): add multi-tab demo with dual BimEngine instances

New demo page with two independent BimEngine instances side by side,
draggable divider for resizing, and proper SDK-layer resize handling.
Replaces the iframe-based approach now that ManagerRegistry is instance-based.
Removes unused panelViewer vite config entry.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

demo/index.html

@@ -171,6 +171,17 @@
             </div>
         </div>
 
+        <!-- 7. 多Tab加载 -->
+        <div class="control-group">
+            <h2>📺 多Tab加载 (Multi-Tab)</h2>
+            <div class="btn-container">
+                <button class="primary" onclick="window.open('./multi-tab.html', '_blank')">打开多Tab视图</button>
+            </div>
+            <div style="margin-top: 8px; font-size: 0.8rem; color: #888;">
+                两个独立面板，各自加载模型，可拖拽分割线调整宽度
+            </div>
+        </div>
+
         <!-- 5. 3D 引擎 -->
         <div class="control-group">
             <h2>🎮 3D 引擎 (Engine3D)</h2>
@@ -178,6 +189,7 @@
                 <button class="primary" onclick="initEngine3D()">初始化引擎</button>
                 <button class="primary" onclick="loadModel()">加载模型</button>
                 <button onclick="switchModel()">切换模型</button>
+                <button onclick="loadCombinedModel()">组合模型</button>
             </div>
             <div class="btn-container" style="margin-top: 8px;">
                 <button onclick="pauseRendering()">暂停渲染</button>
@@ -410,6 +422,38 @@
             }
         }
 
+        /**
+         * 加载组合模型 - 同时加载多个模型
+         */
+        function loadCombinedModel() {
+            if (!engine || !engine.engine) {
+                alert('引擎未创建，请先等待页面加载完成');
+                return;
+            }
+
+            if (!engine.engine.isInitialized()) {
+                alert('请先初始化 3D 引擎！');
+                return;
+            }
+
+            try {
+                const modelUrls = [
+                    'https://lyz-1259524260.cos.ap-guangzhou.myqcloud.com/iflow/models/417664a3-76c8-4d94-9344-1337246a5d4e/',
+                    'https://lyz-1259524260.cos.ap-guangzhou.myqcloud.com/iflow/models/e49a5fd1-3018-4938-9a52-6862b56a190b/'
+                ];
+
+                engine.engine.loadModel(modelUrls, {
+                    position: [0, 0, 0],
+                    rotation: [0, 0, 0],
+                    scale: [1, 1, 1]
+                });
+
+                console.log('✅ 组合模型加载请求已发送:', modelUrls);
+            } catch (error) {
+                console.error('❌ 组合模型加载错误:', error);
+            }
+        }
+
         /**
          * 暂停渲染
          */