Address code review feedback: use IsRemoved flag and add comments

Co-authored-by: RuntimeRascal <2422222+RuntimeRascal@users.noreply.github.com>

Author: Copilot

Src/GhostDraw/Services/DrawingHistory.cs

@@ -76,7 +76,7 @@ public void RecordAction(UIElement element)
                 _elementIdToEntry.Remove(entry.Id);
 
                 // Check if element is still valid (not already removed)
-                if (entry.Element != null)
+                if (!entry.IsRemoved)
                 {
                     _logger.LogInformation("Undo: Removing element ID={Id}, Type={Type}, RemainingActions={Count}",
                         entry.Id, entry.Element.GetType().Name, _undoStack.Count);
@@ -85,7 +85,7 @@ public void RecordAction(UIElement element)
                 else
                 {
                     // Element was already removed (e.g., by eraser), skip to next
-                    _logger.LogDebug("Undo: Skipping null element ID={Id} (already removed)", entry.Id);
+                    _logger.LogDebug("Undo: Skipping removed element ID={Id}", entry.Id);
                 }
             }
 
@@ -112,8 +112,8 @@ public void RemoveFromHistory(UIElement element)
             {
                 if (_elementIdToEntry.TryGetValue(id, out var entry))
                 {
-                    // Mark the entry's element as null so it will be skipped during undo
-                    entry.Element = null;
+                    // Mark the entry as removed so it will be skipped during undo
+                    entry.IsRemoved = true;
                     _elementIdToEntry.Remove(id);
 
                     _logger.LogDebug("Element removed from history: ID={Id}, Type={Type}",
@@ -168,12 +168,14 @@ public void Clear()
     private class HistoryEntry
     {
         public Guid Id { get; }
-        public UIElement? Element { get; set; }
+        public UIElement Element { get; }
+        public bool IsRemoved { get; set; }
 
         public HistoryEntry(Guid id, UIElement element)
         {
             Id = id;
             Element = element;
+            IsRemoved = false;
         }
     }
 }

Src/GhostDraw/Tools/EraserTool.cs

@@ -30,6 +30,9 @@ public class EraserTool(ILogger<EraserTool> logger) : IDrawingTool
     private double _currentThickness = 3.0;
     private readonly HashSet<UIElement> _erasedElements = new();
 
+    // ActionCompleted event required by IDrawingTool but not used by eraser.
+    // Eraser uses ElementErased event instead since it removes existing elements
+    // rather than creating new ones.
     public event EventHandler<DrawingActionCompletedEventArgs>? ActionCompleted;
 
     /// <summary>