router/serve: restore the cfg(unix) gating the listener refactor dropped

[obodnikov]

What does this PR do?

src/router/serve/unix.rs is #![cfg(unix)], and serve.rs used to respect
that. Through v1.5.1 it contained no unix-specific code at all — the socket work
lived entirely inside that gated submodule and serve.rs itself was
platform-neutral.

da92b973 Refactor axum listener setup, fixes #374 moved that work inline
without carrying the gate across. serve.rs now has:

• an unconditional use std::os::unix::net::UnixListener,
• an unconditional systemd_listeners()?,
• make_log_addrs(…, &passed_unix_listeners), whose signature takes
  &[UnixListener],
• both systemd_listeners variants selected on not(target_os = "linux"),
  which includes targets that have no unix sockets at all.

The file therefore no longer compiles off unix. v1.8.2 added Cow for the
map_or inside make_log_addrs, which inherits the same problem.

This puts the gate back:

• the unix-only import, the passed_unix_listeners data flow and
  make_log_addrs go behind #[cfg(unix)];
• both systemd_listeners cfgs gain unix;
• a #[cfg(not(unix))] make_log_addrs keeps the TCP and configured-path
  entries, which is all a target without unix sockets can report.

Rather than adding a non-unix systemd_listeners stub, the non-unix path skips
the call and starts from an empty listener set — socket activation is unix-only
by construction, so there is nothing for a stub to mean.

The unix code path is unchanged. Every added cfg selects on unix exactly what
was selected before: target_os = "linux" already implies unix, and the
narrowed second systemd_listeners variant still applies to macOS and the BSDs.
The narrowing only excludes targets where that variant's return type could not be
constructed anyway.

On verification. CI builds Linux only, where this is a no-op both before and
after, so CI cannot demonstrate the change. It was observed building v1.8.2 for
x86_64-pc-windows-msvc. cargo check and cargo clippy on the unix path are
clean before and after.

Third in the series with #526 and #527.

Checklist

• Code is formatted with nightly cargo fmt and satisfies clippy and
  rustc lints; any allowed lint is justified by an obvious reason or a
  comment.
• Complement compliance changes (new passes or new failures), if any,
  are noted in the description above. — none; the unix path is unchanged.
• Config option changes were made in src/core/config/mod.rs doc
  comments and the regenerated tuwunel-example.toml is committed. — n/a
• User-facing changes are reflected in docs/. — n/a
• I agree that my changes may be licensed under the Apache-2.0 licence
  and my conduct is in line with the Contributor's Covenant and
  Tuwunel's Code of Conduct.