fix: send modification notification from AutoMap.Add() with existing key

Chickensoft.Sync.Tests/src/primitives/AutoMapTest.cs

@@ -133,6 +133,39 @@ public void AddBroadcastsModification()
     log.ShouldBe(["modify"]);
   }
 
+  [Fact]
+  public void AddBroadcastsUpdateForExistingValue()
+  {
+    var map = new AutoMap<int, string> { [1] = "one" };
+    var log = new List<string>();
+    using var binding = map.Bind();
+
+    binding.OnUpdate((key, oldValue, newValue) =>
+      log.Add($"update {key} : {oldValue} -> {newValue}"));
+
+    map.Add(1, "uno");
+
+    map.Count.ShouldBe(1);
+    map[1].ShouldBe("uno");
+    log.ShouldBe(["update 1 : one -> uno"]);
+  }
+
+  [Fact]
+  public void AddBroadcastsModificationForExistingValue()
+  {
+    var map = new AutoMap<int, string> { [1] = "one" };
+    var log = new List<string>();
+    using var binding = map.Bind();
+
+    binding.OnModify(() => log.Add($"modify"));
+
+    map.Add(1, "uno");
+
+    map.Count.ShouldBe(1);
+    map[1].ShouldBe("uno");
+    log.ShouldBe(["modify"]);
+  }
+
   [Fact]
   public void RemoveWithValueBroadcasts()
   {

Chickensoft.Sync/src/primitives/AutoMap.cs

@@ -308,6 +308,7 @@ void IPerform<AddOp>.Perform(in AddOp op)
       _map[key] = value;
 
       _subject.Broadcast(new UpdateBroadcast(key, existing, value));
+      _subject.Broadcast(new ModifyBroadcast());
       return;
     }