拆分导出为两步：遍历(后台线程)+导出(UI线程)

- SectionBoxExporter 新增 ValidateAndTraverse / ExportTraversalToNwd
- ViewModel 拆为两个 await 间有喘息，避免第二次导出时 UI 线程拥塞

src/Core/SectionBoxExporter.cs

@@ -14,6 +14,8 @@ namespace NavisworksTransport.Core
     /// </summary>
     public class SectionBoxExporter
     {
+        private SectionBoxTraversalResult _lastTraversal;
+
         /// <summary>
         /// 导出结果
         /// </summary>
@@ -27,6 +29,99 @@ namespace NavisworksTransport.Core
             public string ErrorMessage { get; set; }
         }
 
+        /// <summary>
+        /// 验证剖面盒并遍历模型树（不含导出，适合后台线程执行）
+        /// </summary>
+        public ExportSectionBoxResult ValidateAndTraverse(Document document)
+        {
+            var result = new ExportSectionBoxResult();
+
+            if (document == null)
+            {
+                result.ErrorMessage = "没有活动的文档！";
+                return result;
+            }
+
+            var viewpoint = document.CurrentViewpoint.Value;
+            var clipPlanes = viewpoint.ClipPlanes;
+
+            if (clipPlanes == null || !clipPlanes.Enabled)
+            {
+                result.ErrorMessage = "当前没有活动的剖面盒！";
+                return result;
+            }
+
+            if (clipPlanes.Mode != ClipPlaneSetMode.Box)
+            {
+                result.ErrorMessage = "请切换为剖面盒模式（Box 模式）。";
+                return result;
+            }
+
+            if (!GetSectionBoxBounds(clipPlanes, out BoundingBox3D bounds))
+            {
+                result.ErrorMessage = "无法获取剖面盒边界。";
+                return result;
+            }
+
+            var traversal = GetObjectsAndHiddenItems(document, bounds);
+            _lastTraversal = traversal;
+            result.ObjectCount = traversal.ObjectsInBox.Count;
+            result.HiddenNodeCount = traversal.ItemsToHide.Count;
+
+            if (traversal.ObjectsInBox.Count == 0)
+            {
+                result.ErrorMessage = "剖面盒内没有找到对象！";
+                return result;
+            }
+
+            result.Success = true;
+            return result;
+        }
+
+        /// <summary>
+        /// 导出遍历结果到 NWD（必须在 UI 线程调用）
+        /// </summary>
+        public ExportSectionBoxResult ExportTraversalToNwd(Document document, string filePath)
+        {
+            var result = new ExportSectionBoxResult();
+
+            if (_lastTraversal == null || _lastTraversal.ObjectsInBox.Count == 0)
+            {
+                result.ErrorMessage = "没有可导出的遍历结果，请先调用 ValidateAndTraverse。";
+                return result;
+            }
+
+            result.ObjectCount = _lastTraversal.ObjectsInBox.Count;
+            result.HiddenNodeCount = _lastTraversal.ItemsToHide.Count;
+
+            LogManager.Info($"[SectionBoxExporter] 剖面盒内找到 {result.ObjectCount} 个对象，可隐藏 {result.HiddenNodeCount} 个节点");
+
+            try
+            {
+                var exportedPath = NwdExportHelper.ExportToNwdWithPrecomputedHiddenItems(
+                    _lastTraversal.ObjectsInBox,
+                    _lastTraversal.ItemsToHide,
+                    filePath,
+                    "剖面盒导出");
+
+                if (string.IsNullOrEmpty(exportedPath) || !File.Exists(exportedPath))
+                {
+                    result.ErrorMessage = "导出失败：未生成文件。";
+                    return result;
+                }
+
+                result.Success = true;
+                result.FilePath = exportedPath;
+                result.FileSize = new FileInfo(exportedPath).Length;
+            }
+            catch (Exception ex)
+            {
+                result.ErrorMessage = $"导出失败：{ex.Message}";
+            }
+
+            return result;
+        }
+
         /// <summary>
         /// 从当前视点导出剖面盒到 NWD（完整业务流程：验证→遍历→导出）
         /// </summary>

src/UI/WPF/ViewModels/LayerManagementViewModel.cs

@@ -1946,23 +1946,31 @@ namespace NavisworksTransport.UI.WPF.ViewModels
                 if (string.IsNullOrEmpty(saveFilePath))
                     return;
 
-                // 委托给业务层：验证剖面盒 → 遍历模型树 → 导出
+                // 第一步：验证剖面盒 + 遍历模型树（后台线程）
                 CurrentOperationText = "正在查找剖面盒内的对象...";
-                var result = await Task.Run(() =>
-                {
-                    var exporter = new SectionBoxExporter();
-                    return exporter.ExportSectionBoxFromActiveView(document, saveFilePath);
-                });
+                var exporter = new SectionBoxExporter();
+                SectionBoxExporter.ExportSectionBoxResult traverseResult = await Task.Run(() =>
+                    exporter.ValidateAndTraverse(document));
 
-                if (result.Success)
+                if (!traverseResult.Success)
+                {
+                    MessageBox.Show(traverseResult.ErrorMessage, "提示", MessageBoxButton.OK, MessageBoxImage.Warning);
+                    return;
+                }
+
+                // 第二步：导出（UI 线程，内部通过 Dispatcher.Invoke 执行）
+                CurrentOperationText = "正在导出...";
+                var exportResult = exporter.ExportTraversalToNwd(document, saveFilePath);
+
+                if (exportResult.Success)
                 {
                     MessageBox.Show(
-                        $"NWD导出成功！\n\n文件: {result.FilePath}\n大小: {result.FileSize / 1024} KB\n对象数: {result.ObjectCount}",
+                        $"NWD导出成功！\n\n文件: {exportResult.FilePath}\n大小: {exportResult.FileSize / 1024} KB\n对象数: {exportResult.ObjectCount}",
                         "导出结果", MessageBoxButton.OK, MessageBoxImage.Information);
                 }
                 else
                 {
-                    MessageBox.Show(result.ErrorMessage, "提示", MessageBoxButton.OK, MessageBoxImage.Warning);
+                    MessageBox.Show(exportResult.ErrorMessage, "提示", MessageBoxButton.OK, MessageBoxImage.Warning);
                 }
             }
             catch (Exception ex)