transport: rename darwin_sockopts to sockopts_macos

The module is gated target_os="macos" (not the broader Darwin/iOS family),
so the name now tracks the cfg and matches the *_macos.rs file convention
(io_macos.rs). Drops the redundant inner #![cfg(target_os="macos")] (the
decl gate already covers it) and corrects a stale top comment that claimed
the module stays visible on Linux — it is macos-decl-gated, so it never was.
Mechanical rename; no logic change.

src/transport/udp/io.rs

@@ -800,7 +800,7 @@ pub use platform::{AsyncUdpSocket, UdpRawSocket};
 ///
 /// Gated to Linux/macOS: the rest of `io.rs` compiles more broadly
 /// (Windows uses `tokio::net::UdpSocket`), but the connected fast path
-/// is libc-syscall + `darwin_sockopts` specific.
+/// is libc-syscall + `sockopts_macos` specific.
 #[cfg(any(target_os = "linux", target_os = "macos"))]
 mod connected {
     // The connected-UDP fast path is infra-ready but not yet wired into
@@ -873,7 +873,7 @@ mod connected {
         set_sockopt_int(raw, libc::SOL_SOCKET, libc::SO_REUSEPORT, 1)?;
 
         #[cfg(target_os = "macos")]
-        crate::transport::udp::darwin_sockopts::apply_udp_socket_tuning(raw, "connected-udp-peer");
+        crate::transport::udp::sockopts_macos::apply_udp_socket_tuning(raw, "connected-udp-peer");
 
         // Buffer sizes — try the FORCE variants first (succeed if we
         // have CAP_NET_ADMIN), then fall back to the ceiling-clamped

src/transport/udp/mod.rs

@@ -6,9 +6,9 @@ use super::{
     DiscoveredPeer, PacketTx, ReceivedPacket, Transport, TransportAddr, TransportError,
     TransportId, TransportState, TransportType,
 };
-#[cfg(target_os = "macos")]
-pub(crate) mod darwin_sockopts;
 pub(crate) mod io;
+#[cfg(target_os = "macos")]
+pub(crate) mod sockopts_macos;
 #[cfg(any(target_os = "linux", target_os = "macos"))]
 pub(crate) use io::open_connected_fd;
 mod stats;

src/transport/udp/sockopts_macos.rs

@@ -1,6 +1,5 @@
-// Applied inside `io::open_connected_fd` (dormant in this PR; see
-// `io.rs`). Linux toolchain only checks gates — keep the module
-// visible on Linux so clippy doesn't lose track.
+// Applied inside `io::open_connected_fd` (see `io.rs`), dormant until
+// the connected-UDP fast path is wired into dispatch — hence the allow.
 #![allow(dead_code)]
 
 //! Darwin UDP socket tuning.
@@ -10,8 +9,6 @@
 //! service type is the one low-cost Darwin hint that can change how the
 //! socket is queued by the host networking stack and Wi-Fi WMM.
 
-#![cfg(target_os = "macos")]
-
 use std::io;
 use std::os::fd::RawFd;
 use std::sync::OnceLock;