Pre-size MultiDictionary in CreateEvaluatedIncludeSnapshotIfRequested to avoid resize allocations

src/Build/Collections/MultiDictionary.cs

@@ -68,6 +68,15 @@ internal MultiDictionary(IEqualityComparer<K> keyComparer)
             _backing = new Dictionary<K, SmallList<V>>(keyComparer);
         }
 
+        /// <summary>
+        /// Constructor taking a specified comparer and initial capacity for the keys.
+        /// Pre-sizing avoids repeated dictionary resizes when the number of keys is known.
+        /// </summary>
+        internal MultiDictionary(int capacity, IEqualityComparer<K> keyComparer)
+        {
+            _backing = new Dictionary<K, SmallList<V>>(capacity, keyComparer);
+        }
+
         /// <summary>
         /// Number of keys
         /// </summary>

src/Build/Instance/ProjectInstance.cs

@@ -3398,7 +3398,7 @@ private void CreateEvaluatedIncludeSnapshotIfRequested(bool keepEvaluationCache,
                 return;
             }
 
-            var multiDictionary = new MultiDictionary<string, ProjectItemInstance>(StringComparer.OrdinalIgnoreCase);
+            var multiDictionary = new MultiDictionary<string, ProjectItemInstance>(items.Count, StringComparer.OrdinalIgnoreCase);
             foreach (var item in items)
             {
                 multiDictionary.Add(item.EvaluatedInclude, projectItemToInstanceMap[item]);