AX Agent transcript 호스트를 컬렉션 기반으로 재구성해 코워크·코드 스트리밍 중 UI 부하를 줄임

- 메시지 영역을 ScrollViewer+StackPanel 직접 조작 구조에서 ListBox+ObservableCollection 기반 transcript 호스트로 전환
- ChatWindow.TranscriptHost 도입으로 transcript 요소 추가·교체·삭제·스크롤 접근을 공용 helper로 정리
- 코워크/코드 실행 중 RenderMessages와 라이브 카드가 MessagePanel.Children 직접 조작에 덜 의존하도록 정리
- MessageBubble, AgentEventRendering, Timeline, UserAsk, ConversationManagement 등 transcript 관련 partial을 새 호스트 구조에 맞게 전환
- 향후 claw-code의 VirtualMessageList 수준 가상화를 적용할 수 있는 기반을 마련
- README와 DEVELOPMENT 문서를 2026-04-08 12:52 (KST) 기준으로 갱신
- 검증: dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify\\ -p:IntermediateOutputPath=obj\\verify\\ (경고 0, 오류 0)

src/AxCopilot/Views/ChatWindow.TranscriptHost.cs

@@ -0,0 +1,101 @@
+using System.Collections.ObjectModel;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+
+namespace AxCopilot.Views;
+
+public partial class ChatWindow
+{
+    private readonly ObservableCollection<UIElement> _transcriptElements = [];
+    private ScrollViewer? _transcriptScrollViewer;
+    private bool _transcriptScrollHooked;
+
+    private void InitializeTranscriptHost()
+    {
+        MessageList.ItemsSource = _transcriptElements;
+        AttachTranscriptScrollChanged();
+        MessageList.Loaded += (_, _) => AttachTranscriptScrollChanged();
+    }
+
+    private void AttachTranscriptScrollChanged()
+    {
+        var scrollViewer = FindVisualChild<ScrollViewer>(MessageList);
+        if (scrollViewer == null || ReferenceEquals(scrollViewer, _transcriptScrollViewer))
+            return;
+
+        if (_transcriptScrollViewer != null && _transcriptScrollHooked)
+        {
+            _transcriptScrollViewer.ScrollChanged -= MessageScroll_ScrollChanged;
+        }
+
+        _transcriptScrollViewer = scrollViewer;
+        _transcriptScrollViewer.ScrollChanged += MessageScroll_ScrollChanged;
+        _transcriptScrollHooked = true;
+    }
+
+    private static T? FindVisualChild<T>(DependencyObject? parent)
+        where T : DependencyObject
+    {
+        if (parent == null)
+            return null;
+
+        var childCount = VisualTreeHelper.GetChildrenCount(parent);
+        for (var i = 0; i < childCount; i++)
+        {
+            var child = VisualTreeHelper.GetChild(parent, i);
+            if (child is T match)
+                return match;
+
+            var descendant = FindVisualChild<T>(child);
+            if (descendant != null)
+                return descendant;
+        }
+
+        return null;
+    }
+
+    private int GetTranscriptElementCount() => _transcriptElements.Count;
+
+    private UIElement? GetTranscriptElementAt(int index)
+    {
+        if (index < 0 || index >= _transcriptElements.Count)
+            return null;
+
+        return _transcriptElements[index];
+    }
+
+    private int GetTranscriptElementIndex(UIElement element) => _transcriptElements.IndexOf(element);
+
+    private bool ContainsTranscriptElement(UIElement element) => _transcriptElements.Contains(element);
+
+    private void AddTranscriptElement(UIElement element) => _transcriptElements.Add(element);
+
+    private void ClearTranscriptElements() => _transcriptElements.Clear();
+
+    private void RemoveTranscriptElement(UIElement element) => _transcriptElements.Remove(element);
+
+    private void RemoveTranscriptElementAt(int index)
+    {
+        if (index < 0 || index >= _transcriptElements.Count)
+            return;
+
+        _transcriptElements.RemoveAt(index);
+    }
+
+    private void ReplaceTranscriptElement(int index, UIElement element)
+    {
+        if (index < 0 || index >= _transcriptElements.Count)
+            return;
+
+        _transcriptElements[index] = element;
+    }
+
+    private double GetTranscriptScrollableHeight() => _transcriptScrollViewer?.ScrollableHeight ?? 0;
+
+    private double GetTranscriptVerticalOffset() => _transcriptScrollViewer?.VerticalOffset ?? 0;
+
+    private void ScrollTranscriptToEnd() => _transcriptScrollViewer?.ScrollToEnd();
+
+    private void ScrollTranscriptToVerticalOffset(double offset) => _transcriptScrollViewer?.ScrollToVerticalOffset(offset);
+}