AX Agent loop 정책을 분리하고 transcript 가상화·성능 계측 구조를 강화한다

- AgentLoop 검증/문서/compact 정책 메서드를 partial 파일로 분리해 loop 본문의 책임을 줄임
- transcript 렌더에 cache pruning과 deferred scrolling을 적용해 긴 세션의 UI 부담을 낮춤
- AgentPerformanceLogService를 추가해 transcript 렌더와 agent loop 실행 요약을 perf 로그로 남김
- README와 DEVELOPMENT 문서에 2026-04-09 10:08 (KST) 기준 구조 개선 및 계측 이력을 반영함
- dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify\\ -p:IntermediateOutputPath=obj\\verify\\ 검증 결과 경고 0개 오류 0개

src/AxCopilot/Views/ChatWindow.TranscriptRendering.cs

@@ -1,5 +1,7 @@
+using System.Diagnostics;
 using System.Windows.Threading;
 using AxCopilot.Models;
+using AxCopilot.Services;
 
 namespace AxCopilot.Views;
 
@@ -18,6 +20,7 @@ public partial class ChatWindow
 
     private void RenderMessages(bool preserveViewport = false)
     {
+        var renderStopwatch = Stopwatch.StartNew();
         var previousScrollableHeight = GetTranscriptScrollableHeight();
         var previousVerticalOffset = GetTranscriptVerticalOffset();
 
@@ -62,10 +65,32 @@ public partial class ChatWindow
 
         if (!TryApplyIncrementalTranscriptRender(renderPlan))
             ApplyFullTranscriptRender(renderPlan);
+        PruneTranscriptElementCache(renderPlan.NewKeys);
 
         _lastRenderedMessageCount = visibleMessages.Count;
         _lastRenderedEventCount = visibleEvents.Count;
         _lastRenderedShowHistory = renderPlan.ShowHistory;
+        renderStopwatch.Stop();
+        if (renderStopwatch.ElapsedMilliseconds >= 24 || _isStreaming)
+        {
+            AgentPerformanceLogService.LogMetric(
+                "transcript",
+                "render_messages",
+                conv.Id,
+                _activeTab ?? "",
+                renderStopwatch.ElapsedMilliseconds,
+                new
+                {
+                    preserveViewport,
+                    streaming = _isStreaming,
+                    lightweight = IsLightweightLiveProgressMode(),
+                    visibleMessages = visibleMessages.Count,
+                    visibleEvents = visibleEvents.Count,
+                    renderedItems = renderPlan.NewKeys.Count,
+                    hiddenCount = renderPlan.HiddenCount,
+                    transcriptElements = GetTranscriptElementCount(),
+                });
+        }
 
         if (!preserveViewport)
         {