fix(netwatch): guard divide-by-zero in poll_recv_noq trace

`UdpSocket::poll_recv_noq` evaluated `meta.len / meta.stride` directly
inside `trace!()` macro arguments. `noq_udp::RecvMeta.stride` is allowed
to be `0` in practice on the GRO path (empty datagrams, kernel falling
back to a non-segmented receive), which crashes the host process with
`attempt to divide by zero` — observed on Windows 11 24H2 and macOS
15.4/26.x against netwatch 0.16.0/0.17.0.

Replace the bare division with `checked_div(...).unwrap_or(0)` so the
trace field reports `0` when `stride == 0` and stays identical
otherwise.

Closes #148.

Author: mkdir700

netwatch/src/udp.rs

@@ -433,7 +433,12 @@ impl UdpSocket {
                         trace!(
                             src = %meta.addr,
                             len = meta.len,
-                            count = meta.len / meta.stride,
+                            // `stride == 0` is reachable in practice on the GRO path
+                            // (e.g. empty datagrams, kernel falling back to a
+                            // non-segmented receive), so guard the division to
+                            // avoid a divide-by-zero panic inside the trace
+                            // macro arguments. See n0-computer/net-tools#148.
+                            count = meta.len.checked_div(meta.stride).unwrap_or(0),
                             dst = %meta.dst_ip.map(|x| x.to_string()).unwrap_or_default(),
                             "UDP recv"
                         );