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.App.Tests/Net/LiveSessionHostTests.cs

@@ -381,12 +381,20 @@ public sealed class LiveSessionHostTests
         public void Send(ReadOnlySpan<byte> datagram) { }
         public void Send(IPEndPoint remote, ReadOnlySpan<byte> datagram) { }
 
-        public byte[]? Receive(TimeSpan timeout, out IPEndPoint? from)
+        public int Receive(
+            Span<byte> destination,
+            TimeSpan timeout,
+            out IPEndPoint? from)
         {
             from = null;
-            return null;
+            return -1;
         }
 
+        public ValueTask<NetReceiveResult> ReceiveAsync(
+            Memory<byte> destination,
+            CancellationToken cancellationToken) =>
+            throw new OperationCanceledException(cancellationToken);
+
         public void Dispose() { }
     }
 }