add singular ReadMessage to System

src/MessageDepot.cs

@@ -24,11 +24,16 @@ internal class MessageDepot
 		return Lookup<TMessage>().Some();
 	}
 
-	public ReadOnlySpan<TMessage> Read<TMessage>() where TMessage : struct
+	public ReadOnlySpan<TMessage> ReadAll<TMessage>() where TMessage : struct
 	{
 		return Lookup<TMessage>().All();
 	}
 
+	public TMessage ReadFirst<TMessage>() where TMessage : struct
+	{
+		return Lookup<TMessage>().First();
+	}
+
 	public void Clear()
 	{
 		foreach (var storage in storages.Values)

src/MessageStorage.cs

@@ -38,6 +38,11 @@ internal class MessageStorage<TMessage> : MessageStorage where TMessage : struct
 		return new ReadOnlySpan<TMessage>(messages, 0, count);
 	}
 
+	public TMessage First()
+	{
+		return messages[0];
+	}
+
 	public override void Clear()
 	{
 		count = 0;

src/System.cs

@@ -38,7 +38,12 @@ public abstract class System : EntityComponentReader
 
 	protected ReadOnlySpan<TMessage> ReadMessages<TMessage>() where TMessage : struct
 	{
-		return MessageDepot.Read<TMessage>();
+		return MessageDepot.ReadAll<TMessage>();
+	}
+
+	protected TMessage ReadMessage<TMessage>() where TMessage : struct
+	{
+		return MessageDepot.ReadFirst<TMessage>();
 	}
 
 	protected bool SomeMessage<TMessage>() where TMessage : struct