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.

src/AcDream.Core.Net/Packets/FragmentAssembler.cs

@@ -95,6 +95,80 @@ public sealed class FragmentAssembler
         return combined;
     }
 
+    /// <summary>
+    /// Production borrowed-memory path. A complete single-fragment message
+    /// is returned as a view into the current datagram. Multi-fragment
+    /// payloads are copied only because they must survive that datagram's
+    /// pooled lifetime.
+    /// </summary>
+    internal bool TryIngest(
+        in BorrowedMessageFragment fragment,
+        out ReadOnlyMemory<byte> message,
+        out ushort messageQueue)
+    {
+        MessageFragmentHeader header = fragment.Header;
+        message = ReadOnlyMemory<byte>.Empty;
+        messageQueue = 0;
+
+        if (header.Count == 1 && header.Index == 0)
+        {
+            message = fragment.Payload;
+            messageQueue = header.Queue;
+            return true;
+        }
+
+        if (!_inFlight.TryGetValue(
+                header.Sequence,
+                out PartialMessage? partial))
+        {
+            partial = new PartialMessage(
+                header.Count,
+                header.Queue);
+            _inFlight[header.Sequence] = partial;
+        }
+        else if (partial.TotalFragments != header.Count
+                 || partial.Queue != header.Queue)
+        {
+            // Same sequence with conflicting identity is malformed. Preserve
+            // the first accepted partial instead of corrupting its layout.
+            return false;
+        }
+
+        if (partial.Fragments[header.Index] is null)
+        {
+            partial.Fragments[header.Index] =
+                fragment.Payload.ToArray();
+            partial.ReceivedCount++;
+        }
+
+        if (partial.ReceivedCount < partial.TotalFragments)
+            return false;
+
+        int totalBytes = 0;
+        for (int index = 0;
+             index < partial.TotalFragments;
+             index++)
+        {
+            totalBytes += partial.Fragments[index]!.Length;
+        }
+
+        var combined = new byte[totalBytes];
+        int offset = 0;
+        for (int index = 0;
+             index < partial.TotalFragments;
+             index++)
+        {
+            byte[] payload = partial.Fragments[index]!;
+            payload.CopyTo(combined, offset);
+            offset += payload.Length;
+        }
+
+        _inFlight.Remove(header.Sequence);
+        message = combined;
+        messageQueue = partial.Queue;
+        return true;
+    }
+
     /// <summary>Discard all in-flight partial messages.</summary>
     public void DropAll() => _inFlight.Clear();