perf(net): frame recurring sends in place

Write normal game-message and ACK packet framing directly into bounded stack spans, hash fragments without materialization, and send the populated slice to the socket. Preserve exact wire bytes and ISAAC failure ordering with differential and zero-allocation tests.

tests/AcDream.Core.Net.Tests/Messages/CharacterEnterWorldTests.cs

@@ -149,4 +149,41 @@ public class GameMessageFragmentTests
         Assert.Equal(original.Header.Count,    reparsed.Value.Header.Count);
         Assert.Equal(original.Payload, reparsed.Value.Payload);
     }
+
+    [Fact]
+    public void WriteSingleFragment_MatchesOwnedBuildAndSerialize()
+    {
+        byte[] message = [1, 2, 3, 4, 5, 6];
+        MessageFragment owned =
+            GameMessageFragment.BuildSingleFragment(
+                fragmentSequence: 99,
+                queue: GameMessageGroup.ControlQueue,
+                gameMessageBytes: message);
+        byte[] expected = GameMessageFragment.Serialize(owned);
+        Span<byte> destination = stackalloc byte[
+            MessageFragmentHeader.MaxFragmentSize];
+
+        int written = GameMessageFragment.WriteSingleFragment(
+            destination,
+            fragmentSequence: 99,
+            queue: GameMessageGroup.ControlQueue,
+            gameMessageBytes: message);
+
+        Assert.Equal(expected.Length, written);
+        Assert.Equal(expected, destination.Slice(0, written).ToArray());
+    }
+
+    [Fact]
+    public void WriteSingleFragment_ShortDestination_Throws()
+    {
+        byte[] message = [1, 2, 3, 4];
+        Assert.Throws<ArgumentException>(
+            () => GameMessageFragment.WriteSingleFragment(
+                new byte[
+                    MessageFragmentHeader.Size
+                    + message.Length - 1],
+                fragmentSequence: 1,
+                queue: GameMessageGroup.UIQueue,
+                gameMessageBytes: message));
+    }
 }