구조 개선: transcript 지연 가상화와 tool executor 분리 적용

이번 변경은 claude-code 기준 구조 강건성을 높이기 위한 리팩터링입니다.

핵심 수정 사항:

- AX Agent transcript를 TranscriptVisualItem/TranscriptVisualHost 기반 지연 materialization 구조로 전환해 MessageList 가상화 기반을 강화했습니다.

- StreamingToolExecutionCoordinator를 IToolExecutionCoordinator 뒤로 분리해 AgentLoopService가 구체 구현에 직접 묶이지 않도록 정리했습니다.

- 라이브 진행 카드 렌더를 ChatWindow.LiveProgressPresentation partial로 이동해 ChatWindow.xaml.cs의 책임을 더 줄였습니다.

- 기존 메시지 bubble 조립 로직을 유지하면서 transcript host가 필요 시점에 bubble을 만들 수 있도록 helper 경로를 추가했습니다.

검증 결과:

- 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

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Windows;
 using System.Windows.Controls;
@@ -7,7 +9,8 @@ namespace AxCopilot.Views;
 
 public partial class ChatWindow
 {
-    private readonly ObservableCollection<UIElement> _transcriptElements = [];
+    private readonly ObservableCollection<TranscriptVisualItem> _transcriptElements = [];
+    private readonly Dictionary<UIElement, TranscriptVisualItem> _transcriptElementMap = new();
     private ScrollViewer? _transcriptScrollViewer;
     private bool _transcriptScrollHooked;
 
@@ -55,6 +58,8 @@ public partial class ChatWindow
         return null;
     }
 
+    private static string CreateTranscriptElementKey() => $"ui_{Guid.NewGuid():N}";
+
     private int GetTranscriptElementCount() => _transcriptElements.Count;
 
     private UIElement? GetTranscriptElementAt(int index)
@@ -62,24 +67,73 @@ public partial class ChatWindow
         if (index < 0 || index >= _transcriptElements.Count)
             return null;
 
-        return _transcriptElements[index];
+        return _transcriptElements[index].GetOrCreateElement();
     }
 
-    private int GetTranscriptElementIndex(UIElement element) => _transcriptElements.IndexOf(element);
+    private int GetTranscriptElementIndex(UIElement element)
+    {
+        if (_transcriptElementMap.TryGetValue(element, out var item))
+            return _transcriptElements.IndexOf(item);
 
-    private bool ContainsTranscriptElement(UIElement element) => _transcriptElements.Contains(element);
+        for (var i = 0; i < _transcriptElements.Count; i++)
+        {
+            var candidate = _transcriptElements[i];
+            if (!candidate.IsMaterialized)
+                continue;
 
-    private void AddTranscriptElement(UIElement element) => _transcriptElements.Add(element);
+            if (ReferenceEquals(candidate.Element, element))
+            {
+                _transcriptElementMap[element] = candidate;
+                return i;
+            }
+        }
 
-    private void ClearTranscriptElements() => _transcriptElements.Clear();
+        return -1;
+    }
 
-    private void RemoveTranscriptElement(UIElement element) => _transcriptElements.Remove(element);
+    private bool ContainsTranscriptElement(UIElement element) => GetTranscriptElementIndex(element) >= 0;
+
+    private TranscriptVisualItem WrapTranscriptElement(string key, UIElement element)
+    {
+        var item = new TranscriptVisualItem(key, element);
+        _transcriptElementMap[element] = item;
+        return item;
+    }
+
+    private void AddTranscriptElement(UIElement element)
+        => _transcriptElements.Add(WrapTranscriptElement(CreateTranscriptElementKey(), element));
+
+    private void AddDeferredTranscriptElement(string key, Func<UIElement> elementFactory)
+    {
+        TranscriptVisualItem? item = null;
+        item = new TranscriptVisualItem(
+            key,
+            elementFactory,
+            element => _transcriptElementMap[element] = item!);
+        _transcriptElements.Add(item);
+    }
+
+    private void ClearTranscriptElements()
+    {
+        _transcriptElements.Clear();
+        _transcriptElementMap.Clear();
+    }
+
+    private void RemoveTranscriptElement(UIElement element)
+    {
+        var index = GetTranscriptElementIndex(element);
+        if (index >= 0)
+            RemoveTranscriptElementAt(index);
+    }
 
     private void RemoveTranscriptElementAt(int index)
     {
         if (index < 0 || index >= _transcriptElements.Count)
             return;
 
+        var item = _transcriptElements[index];
+        if (item.Element != null)
+            _transcriptElementMap.Remove(item.Element);
         _transcriptElements.RemoveAt(index);
     }
 
@@ -88,7 +142,11 @@ public partial class ChatWindow
         if (index < 0 || index >= _transcriptElements.Count)
             return;
 
-        _transcriptElements[index] = element;
+        var previous = _transcriptElements[index];
+        if (previous.Element != null)
+            _transcriptElementMap.Remove(previous.Element);
+
+        _transcriptElements[index] = WrapTranscriptElement(CreateTranscriptElementKey(), element);
     }
 
     private double GetTranscriptScrollableHeight() => _transcriptScrollViewer?.ScrollableHeight ?? 0;