Turn transport/udp.rs into a udp/ directory module: the cross-platform core
(UdpTransport, is_transient_io, spawn_data_punch, the Transport trait impl) stays
in mod.rs; the platform batched-I/O backends move to udp/{linux,windows,apple}.rs.
The trait impl is kept whole -- its per-OS send_batch/send_gso/recv_batch methods
become cfg-gated one-line delegators to pub(super) free fns that take &UdpTransport
(byte-identical bodies, self -> t). transport/mod.rs is unchanged (re-exports still
resolve; udp/mod.rs re-exports windows::send_uso_all). No behavior change.
Module gates: linux = any(linux, android) (Android uses sendmmsg/recvmmsg via its
bionic binding); windows = windows (USO); apple = all(unix, not(any(linux,android)))
(recvmsg_x on Darwin, recv-loop on BSD). GSO stays linux-only.
Verified on all four target families from clean HEAD snapshots: Linux clippy
(quic + no-default, -D warnings) + full test; Windows clippy (both) + test --lib
(156); macOS check (apple recvmsg_x path); aarch64-linux-android check (android_mmsg).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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