perf(net): borrow inbound packet storage

Decode headers, optional fields, fragments, and single-fragment messages directly over pooled datagrams. Copy only fragment state that crosses a datagram lifetime, preserve synchronous dispatch and ACK ordering, and lock the path to the owned decoder with differential and zero-allocation tests.

tests/AcDream.Core.Net.Tests/Packets/MessageFragmentTests.cs

@@ -75,6 +75,42 @@ public class MessageFragmentTests
         Assert.Equal(0, consumed);
     }
 
+    [Fact]
+    public void TryParse_ZeroFragmentCount_ReturnsNull()
+    {
+        var header = new MessageFragmentHeader
+        {
+            TotalSize = MessageFragmentHeader.Size,
+            Count = 0,
+            Index = 0,
+        };
+        byte[] buf = new byte[MessageFragmentHeader.Size];
+        header.Pack(buf);
+
+        var (frag, consumed) = MessageFragment.TryParse(buf);
+
+        Assert.Null(frag);
+        Assert.Equal(0, consumed);
+    }
+
+    [Fact]
+    public void TryParse_IndexOutsideFragmentCount_ReturnsNull()
+    {
+        var header = new MessageFragmentHeader
+        {
+            TotalSize = MessageFragmentHeader.Size,
+            Count = 2,
+            Index = 2,
+        };
+        byte[] buf = new byte[MessageFragmentHeader.Size];
+        header.Pack(buf);
+
+        var (frag, consumed) = MessageFragment.TryParse(buf);
+
+        Assert.Null(frag);
+        Assert.Equal(0, consumed);
+    }
+
     [Fact]
     public void TryParse_ConsumesExactlyOneFragment_LeavesRemainderForCaller()
     {