The quic and wt (WebTransport) listeners wrap the raw UDP socket with
net.PacketConn metrics/stats/admission/limiter wrappers before handing
it to quic-go. WrapPacketConn returns a plain net.PacketConn, which
hides the underlying *net.UDPConn. quic-go probes the conn for
OOBCapablePacketConn (SyscallConn, SetReadBuffer, ReadMsgUDP,
WriteMsgUDP); behind the wrapper that probe fails, so quic-go
disables its kernel offloads and cannot size its receive buffer.
Switch the non-cipher path of both listeners to WrapUDPConn variants,
which forward SyscallConn/SetReadBuffer/ReadMsgUDP/WriteMsgUDP down to
the underlying *net.UDPConn, so quic-go sees an OOBCapablePacketConn
and keeps GSO/GRO + buffer sizing. The same metrics/stats/admission/
limiting still apply.
The quic listener's cipher path keeps the net.PacketConn wrappers,
since CipherPacketConn is not OOB-capable.
+48% throughput (1.80 → 2.69 Gbit/s single-stream, loopback iperf3).
The quic-go receive-buffer warning is gone.
