perf(net): own one pooled async receive

Replace timeout-polled in-world UDP receives with one cancellable caller-buffered socket operation. Transfer only right-sized pooled datagrams through the FIFO, return every ownership edge deterministically, and send caller spans without a transport copy while preserving handshake pacing and ACK order.

tests/AcDream.Core.Net.Tests/NetClientTests.cs

@@ -96,4 +96,50 @@ public class NetClientTests
         Assert.Null(result);
         Assert.Null(from);
     }
+
+    [Fact]
+    public async Task ReceiveAsync_LoopbackWritesIntoCallerOwnedMemory()
+    {
+        using var serverSide =
+            new NetClient(new IPEndPoint(IPAddress.Loopback, 0));
+        using var clientSide =
+            new NetClient(new IPEndPoint(
+                IPAddress.Loopback,
+                serverSide.LocalEndPoint.Port));
+        byte[] outbound = Enumerable.Range(0, 1_500)
+            .Select(static value => (byte)value)
+            .ToArray();
+        byte[] destination = new byte[2_048];
+        using var timeout =
+            new CancellationTokenSource(TimeSpan.FromSeconds(2));
+
+        clientSide.Send(outbound);
+        NetReceiveResult result = await serverSide.ReceiveAsync(
+            destination,
+            timeout.Token);
+
+        Assert.Equal(outbound.Length, result.Length);
+        Assert.Equal(
+            outbound,
+            destination.AsSpan(0, result.Length).ToArray());
+        Assert.Equal(
+            clientSide.LocalEndPoint.Port,
+            result.RemoteEndPoint.Port);
+    }
+
+    [Fact]
+    public async Task ReceiveAsync_IdleCancellationStopsOutstandingReceive()
+    {
+        using var client =
+            new NetClient(new IPEndPoint(IPAddress.Loopback, 1));
+        byte[] destination = new byte[32];
+        using var cancellation = new CancellationTokenSource();
+
+        ValueTask<NetReceiveResult> receive =
+            client.ReceiveAsync(destination, cancellation.Token);
+        cancellation.Cancel();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(
+            async () => await receive);
+    }
 }