AX Agent transcript 렌더 계획/적용 구조를 분리해 유지보수성을 높이고 문서를 갱신한다

- ChatWindow transcript 렌더를 planner/execution 단계로 분리해 RenderMessages 오케스트레이션을 단순화함
- TranscriptRenderPlanner/TranscriptRenderExecution partial을 추가해 planning과 host 적용 책임을 나눔
- README와 DEVELOPMENT 문서에 2026-04-09 09:37 (KST) 기준 구조 개선 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.TranscriptRenderPlanner.cs

@@ -0,0 +1,65 @@
+using AxCopilot.Models;
+
+namespace AxCopilot.Views;
+
+public partial class ChatWindow
+{
+    private sealed class TranscriptRenderPlan
+    {
+        public required List<(string Key, DateTime Timestamp, int Order, Action Render)> VisibleTimeline { get; init; }
+        public required List<string> NewKeys { get; init; }
+        public required bool ShowHistory { get; init; }
+        public required int HiddenCount { get; init; }
+        public required bool CanIncremental { get; init; }
+        public required int PreviousLiveCount { get; init; }
+        public required int PreviousStableCount { get; init; }
+    }
+
+    private TranscriptRenderPlan BuildTranscriptRenderPlan(
+        ChatConversation conv,
+        IReadOnlyList<ChatMessage> visibleMessages,
+        IReadOnlyList<ChatExecutionEvent> visibleEvents)
+    {
+        var orderedTimeline = BuildTimelineRenderActions(visibleMessages, visibleEvents);
+        var effectiveRenderLimit = GetActiveTimelineRenderLimit();
+        var hiddenCount = Math.Max(0, orderedTimeline.Count - effectiveRenderLimit);
+        var visibleTimeline = hiddenCount > 0
+            ? orderedTimeline.GetRange(hiddenCount, orderedTimeline.Count - hiddenCount)
+            : orderedTimeline;
+
+        var newKeys = new List<string>(visibleTimeline.Count);
+        foreach (var item in visibleTimeline)
+            newKeys.Add(item.Key);
+
+        var hasExternalChildren = _agentLiveContainer != null && ContainsTranscriptElement(_agentLiveContainer);
+        var expectedChildCount = _lastRenderedTimelineKeys.Count + (_lastRenderedHiddenCount > 0 ? 1 : 0);
+        var canIncremental = !hasExternalChildren
+            && _lastRenderedTimelineKeys.Count > 0
+            && newKeys.Count >= _lastRenderedTimelineKeys.Count
+            && _lastRenderedHiddenCount == hiddenCount
+            && GetTranscriptElementCount() == expectedChildCount;
+
+        var previousLiveCount = 0;
+        if (canIncremental)
+        {
+            for (var i = _lastRenderedTimelineKeys.Count - 1; i >= 0; i--)
+            {
+                if (_lastRenderedTimelineKeys[i].StartsWith("_live_", StringComparison.Ordinal))
+                    previousLiveCount++;
+                else
+                    break;
+            }
+        }
+
+        return new TranscriptRenderPlan
+        {
+            VisibleTimeline = visibleTimeline,
+            NewKeys = newKeys,
+            ShowHistory = conv.ShowExecutionHistory,
+            HiddenCount = hiddenCount,
+            CanIncremental = canIncremental,
+            PreviousLiveCount = previousLiveCount,
+            PreviousStableCount = _lastRenderedTimelineKeys.Count - previousLiveCount,
+        };
+    }
+}