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/FragmentAssemblerTests.cs

@@ -124,4 +124,81 @@ public class FragmentAssemblerTests
         assembler.DropAll();
         Assert.Equal(0, assembler.PartialCount);
     }
+
+    [Fact]
+    public void TryIngest_BorrowedSingleFragment_ReturnsOriginalMemory()
+    {
+        var assembler = new FragmentAssembler();
+        byte[] payload = [1, 2, 3];
+        var fragment = new BorrowedMessageFragment(
+            MakeFrag(10, 1, 0, payload, 11).Header,
+            payload);
+
+        bool complete = assembler.TryIngest(
+            fragment,
+            out ReadOnlyMemory<byte> message,
+            out ushort queue);
+        payload[1] = 0xAA;
+
+        Assert.True(complete);
+        Assert.Equal(11, queue);
+        Assert.Equal(0xAA, message.Span[1]);
+        Assert.Equal(0, assembler.PartialCount);
+    }
+
+    [Fact]
+    public void TryIngest_BorrowedMultiFragment_CopiesAcrossDatagrams()
+    {
+        var assembler = new FragmentAssembler();
+        byte[] firstPayload = [1, 2];
+        byte[] secondPayload = [3, 4];
+        var first = new BorrowedMessageFragment(
+            MakeFrag(20, 2, 0, firstPayload, 12).Header,
+            firstPayload);
+        var second = new BorrowedMessageFragment(
+            MakeFrag(20, 2, 1, secondPayload, 12).Header,
+            secondPayload);
+
+        Assert.False(assembler.TryIngest(
+            first,
+            out _,
+            out _));
+        firstPayload[0] = 0xFF;
+        Assert.True(assembler.TryIngest(
+            second,
+            out ReadOnlyMemory<byte> message,
+            out ushort queue));
+        secondPayload[0] = 0xEE;
+
+        Assert.Equal(12, queue);
+        Assert.Equal(new byte[] { 1, 2, 3, 4 }, message.ToArray());
+        Assert.Equal(0, assembler.PartialCount);
+    }
+
+    [Fact]
+    public void TryIngest_ConflictingBorrowedIdentity_PreservesOriginalPartial()
+    {
+        var assembler = new FragmentAssembler();
+        var first = new BorrowedMessageFragment(
+            MakeFrag(30, 2, 0, [1], 13).Header,
+            new byte[] { 1 });
+        var conflict = new BorrowedMessageFragment(
+            MakeFrag(30, 3, 1, [9], 14).Header,
+            new byte[] { 9 });
+        var completion = new BorrowedMessageFragment(
+            MakeFrag(30, 2, 1, [2], 13).Header,
+            new byte[] { 2 });
+
+        Assert.False(assembler.TryIngest(first, out _, out _));
+        Assert.False(assembler.TryIngest(conflict, out _, out _));
+        Assert.Equal(1, assembler.PartialCount);
+        Assert.True(assembler.TryIngest(
+            completion,
+            out ReadOnlyMemory<byte> message,
+            out ushort queue));
+
+        Assert.Equal(13, queue);
+        Assert.Equal(new byte[] { 1, 2 }, message.ToArray());
+        Assert.Equal(0, assembler.PartialCount);
+    }
 }