슬래시 팝업 UX 개선: 스크롤/선택 이동 재렌더링 제거

- ChatWindow slash 이동 경로를 RenderSlashPage 기반에서 선택 하이라이트 갱신 기반으로 전환

- 휠/방향키/Home/End 이동 시 UpdateSlashSelectionVisualState + EnsureSlashSelectionVisible 호출로 통일

- 항목 hover 시에도 동일 선택 상태 동기화 경로를 사용해 체감 일관성 개선

- README.md, docs/DEVELOPMENT.md에 2026-04-04 14:01(KST) 기준 이력 반영

- 검증: dotnet build 0경고/0오류, 관련 필터 테스트 84건 통과

src/AxCopilot/Views/ChatWindow.xaml.cs

@@ -5315,7 +5315,6 @@ public partial class ChatWindow : Window
             var skillAvailable = skillDef?.IsAvailable ?? true;
 
             var absoluteIndex = i;
-            var isSelected = absoluteIndex == _slashPalette.SelectedIndex;
             var hoverBrushItem = TryFindResource("ItemHoverBackground") as Brush ?? Brushes.LightGray;
             var itemBg = TryFindResource("ItemBackground") as Brush ?? Brushes.Transparent;
             var borderBrush = TryFindResource("BorderColor") as Brush ?? Brushes.Gray;
@@ -5325,8 +5324,8 @@ public partial class ChatWindow : Window
 
             var item = new Border
             {
-                Background = isSelected ? hoverBrushItem : itemBg,
-                BorderBrush = isSelected ? accent : borderBrush,
+                Background = Brushes.Transparent,
+                BorderBrush = borderBrush,
                 BorderThickness = new Thickness(1),
                 CornerRadius = new CornerRadius(10),
                 Padding = new Thickness(10, 7, 10, 7),
@@ -5440,14 +5439,9 @@ public partial class ChatWindow : Window
                 item.MouseEnter += (_, _) =>
                 {
                     _slashPalette.SelectedIndex = absoluteIndex;
-                    item.Background = hoverBrushItem;
-                    item.BorderBrush = accent;
-                };
-                item.MouseLeave += (_, _) =>
-                {
-                    item.Background = absoluteIndex == _slashPalette.SelectedIndex ? hoverBrushItem : itemBg;
-                    item.BorderBrush = absoluteIndex == _slashPalette.SelectedIndex ? accent : borderBrush;
+                    UpdateSlashSelectionVisualState();
                 };
+                item.MouseLeave += (_, _) => UpdateSlashSelectionVisualState();
                 item.MouseLeftButtonDown += (_, _) =>
                 {
                     _slashPalette.SelectedIndex = absoluteIndex;
@@ -5499,6 +5493,7 @@ public partial class ChatWindow : Window
             SlashPopupFooter.Text = $"Enter 실행 · ↑↓/PgUp/PgDn 이동 · Home/End · Esc 닫기 · 표시 {visibleCommandCount + visibleSkillCount}/{total}";
         }
 
+        UpdateSlashSelectionVisualState();
         EnsureSlashSelectionVisible();
     }
 
@@ -5546,7 +5541,8 @@ public partial class ChatWindow : Window
         for (var i = 0; i < steps; i++)
             MoveSlashSelection(direction);
 
-        RenderSlashPage();
+        UpdateSlashSelectionVisualState();
+        EnsureSlashSelectionVisible();
     }
 
     /// <summary>키보드로 선택된 슬래시 아이템을 실행합니다.</summary>
@@ -5591,6 +5587,28 @@ public partial class ChatWindow : Window
             SlashScrollViewer.ScrollToVerticalOffset(SlashScrollViewer.VerticalOffset + (bounds.Bottom - SlashScrollViewer.ViewportHeight) + 8);
     }
 
+    private void UpdateSlashSelectionVisualState()
+    {
+        if (_slashVisibleItemByAbsoluteIndex.Count == 0)
+            return;
+
+        var selectedIndex = _slashPalette.SelectedIndex;
+        var itemBg = TryFindResource("ItemBackground") as Brush ?? Brushes.Transparent;
+        var hoverBrushItem = TryFindResource("ItemHoverBackground") as Brush ?? Brushes.LightGray;
+        var borderBrush = TryFindResource("BorderColor") as Brush ?? Brushes.Gray;
+        var accent = TryFindResource("AccentColor") as Brush ?? Brushes.Blue;
+
+        foreach (var (absoluteIndex, element) in _slashVisibleItemByAbsoluteIndex)
+        {
+            if (element is not Border border)
+                continue;
+
+            var selected = absoluteIndex == selectedIndex;
+            border.Background = selected ? hoverBrushItem : itemBg;
+            border.BorderBrush = selected ? accent : borderBrush;
+        }
+    }
+
     /// <summary>슬래시 명령어 즐겨찾기를 토글하고 설정을 저장합니다.</summary>
     private void ToggleSlashFavorite(string cmd)
     {
@@ -10881,14 +10899,16 @@ public partial class ChatWindow : Window
             {
                 var visible = GetVisibleSlashOrderedIndices();
                 _slashPalette.SelectedIndex = visible.Count > 0 ? visible[0] : GetFirstVisibleSlashIndex(_slashPalette.Matches);
-                RenderSlashPage();
+                UpdateSlashSelectionVisualState();
+                EnsureSlashSelectionVisible();
                 e.Handled = true;
             }
             else if (e.Key == Key.End)
             {
                 var visible = GetVisibleSlashOrderedIndices();
                 _slashPalette.SelectedIndex = visible.Count > 0 ? visible[^1] : GetFirstVisibleSlashIndex(_slashPalette.Matches);
-                RenderSlashPage();
+                UpdateSlashSelectionVisualState();
+                EnsureSlashSelectionVisible();
                 e.Handled = true;
             }
             else if (e.Key == Key.Enter && _slashPalette.SelectedIndex >= 0)