feat(windows): add windows-service crate + SCM dispatcher scaffold

Lets numa.exe act as a real Windows service registered with the SCM,
replacing the HKLM\...\Run login-time autostart that runs in the user
session without stderr capture.

- New `numa::windows_service` module (cfg(windows)) wraps Mullvad's
  `windows-service` crate: registers with SCM, reports Running, handles
  Stop/Shutdown, reports Stopped.
- `numa.exe --service` is the entry point SCM uses
  (`sc create … binPath="numa.exe --service"`); interactive invocations
  are unchanged.
- Dep is gated `[target.'cfg(windows)'.dependencies]` — zero impact on
  macOS/Linux builds or binary size.

Scaffold only. The service currently blocks on an mpsc channel until
Stop arrives; the actual serve loop will hook in once main.rs's inline
server body is extracted into `numa::serve(config_path)` in a follow-up.
This lets `sc start Numa` / `sc stop Numa` be verified end to end today.

src/lib.rs

@@ -28,6 +28,9 @@ pub mod system_dns;
 pub mod tls;
 pub mod wire;
 
+#[cfg(windows)]
+pub mod windows_service;
+
 #[cfg(test)]
 pub(crate) mod testutil;