fix(client): remove non-Windows tunnel stub

The Windows client now fails fast outside Windows before tunnel setup because
its real MVP runtime depends on TAP-Windows6 and Windows route protection. The
old non-Windows runtime branch still built a control-event loop and printed
that TAP pumping was not wired yet, which described an obsolete smoke-test path
rather than current behavior.

Replace that branch with an unreachable guard and compile the related runtime
formatting/status helpers only for Windows or unit tests. Non-Windows builds
still type-check the client, but the runtime behavior is now only the explicit
unsupported-platform error.

Test Plan:
- cargo fmt
- cargo test -p lanparty-client-win
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace

Refs: MVP Windows client runtime gate

crates/lanparty-client-win/src/main.rs

@@ -1,8 +1,11 @@
+#[cfg(any(windows, test))]
+use std::collections::BTreeMap;
+#[cfg(any(windows, test))]
 use std::sync::{
     Arc,
     atomic::{AtomicBool, AtomicU32, Ordering},
 };
-use std::{collections::BTreeMap, fs, net::IpAddr, path::PathBuf};
+use std::{fs, net::IpAddr, path::PathBuf};
 #[cfg(windows)]
 use std::{sync::mpsc, thread, time::Duration};
 
@@ -21,12 +24,13 @@ use lanparty_client_route::{
 #[cfg(windows)]
 use lanparty_client_tap::TapAdapter;
 use lanparty_client_tap::TapAdapterInfo;
-use lanparty_ctrl::{ControlMessage, PeerInfo, Role, RoomCode};
+use lanparty_ctrl::RoomCode;
+#[cfg(any(windows, test))]
+use lanparty_ctrl::{ControlMessage, PeerInfo, Role};
 use lanparty_net::RelayEndpoint;
-use lanparty_obs::{
-    ClientDiagnostics, DropReason, FrameAction, FrameDirection, FrameLog, RelayDiagnostics,
-    TapDiagnostics, UserDiagnostic, UserDiagnosticLevel,
-};
+#[cfg(any(windows, test))]
+use lanparty_obs::{ClientDiagnostics, RelayDiagnostics, UserDiagnostic, UserDiagnosticLevel};
+use lanparty_obs::{DropReason, FrameAction, FrameDirection, FrameLog, TapDiagnostics};
 use lanparty_proto::{EthernetFrame, MacAddr};
 
 #[cfg(windows)]
@@ -371,30 +375,8 @@ async fn run_client(
 }
 
 #[cfg(not(windows))]
-async fn run_client(session: &ClientSession) -> Result<()> {
-    let relay_status = ClientRelayStatus::from_welcome(session);
-    open_tap_adapter(session);
-    print_and_report_client_diagnostics(
-        session,
-        &client_diagnostics_snapshot(
-            session,
-            false,
-            &relay_status,
-            TapDiagnostics::new(false, None, None, None),
-        ),
-    )
-    .await;
-    println!("TAP frame pump and route pinning are not wired yet; press Ctrl-C to stop");
-
-    let control_events = run_control_event_log(session, relay_status);
-    tokio::pin!(control_events);
-    let shutdown = tokio::signal::ctrl_c();
-    tokio::pin!(shutdown);
-
-    tokio::select! {
-        result = &mut control_events => result,
-        result = &mut shutdown => result.context("failed to wait for Ctrl-C"),
-    }
+async fn run_client(_session: &ClientSession) -> Result<()> {
+    unreachable!("ensure_supported_platform rejects non-Windows before tunnel setup")
 }
 
 #[cfg(windows)]
@@ -678,6 +660,7 @@ fn format_tap_adapter_list(adapters: &[TapAdapterInfo]) -> Vec<String> {
     lines
 }
 
+#[cfg(windows)]
 fn client_diagnostics_snapshot(
     session: &ClientSession,
     route_pinned: bool,
@@ -705,6 +688,7 @@ fn tap_diagnostics_with_ip(base: &TapDiagnostics, ip: Option<IpAddr>) -> TapDiag
     TapDiagnostics::new(base.adapter_found(), base.mac(), base.mtu(), ip)
 }
 
+#[cfg(windows)]
 fn print_client_diagnostics(diagnostics: &ClientDiagnostics) {
     println!("{}", format_client_diagnostics(diagnostics));
     for diagnostic in diagnostics.user_diagnostics() {
@@ -712,6 +696,7 @@ fn print_client_diagnostics(diagnostics: &ClientDiagnostics) {
     }
 }
 
+#[cfg(windows)]
 async fn print_and_report_client_diagnostics(
     session: &ClientSession,
     diagnostics: &ClientDiagnostics,
@@ -722,6 +707,7 @@ async fn print_and_report_client_diagnostics(
     }
 }
 
+#[cfg(any(windows, test))]
 fn format_client_diagnostics(diagnostics: &ClientDiagnostics) -> String {
     let stats = diagnostics.stats();
     format!(
@@ -793,6 +779,7 @@ fn client_frame_log_line(
     )
 }
 
+#[cfg(any(windows, test))]
 const fn yes_no(value: bool) -> &'static str {
     if value { "yes" } else { "no" }
 }
@@ -805,14 +792,17 @@ fn gateway_status_label(gateway_connected: bool, gateway_peer_id: Option<u32>) -
     }
 }
 
+#[cfg(any(windows, test))]
 fn optional_label<T: std::fmt::Display>(value: Option<T>) -> String {
     value.map_or_else(|| "unknown".to_string(), |value| value.to_string())
 }
 
+#[cfg(any(windows, test))]
 fn optional_millis_label(value: Option<u64>) -> String {
     value.map_or_else(|| "unknown".to_string(), |value| format!("{value} ms"))
 }
 
+#[cfg(any(windows, test))]
 fn format_user_diagnostic(diagnostic: &UserDiagnostic) -> String {
     match diagnostic.level() {
         UserDiagnosticLevel::Info => diagnostic.message().to_owned(),
@@ -821,6 +811,7 @@ fn format_user_diagnostic(diagnostic: &UserDiagnostic) -> String {
     }
 }
 
+#[cfg(windows)]
 async fn run_control_event_log(
     session: &ClientSession,
     relay_status: ClientRelayStatus,
@@ -842,12 +833,14 @@ fn format_control_event(event: &ControlMessage) -> String {
     ControlEventFormatter::default().format(event, &relay_status)
 }
 
+#[cfg(any(windows, test))]
 #[derive(Debug, Clone)]
 struct ClientRelayStatus {
     gateway_connected: Arc<AtomicBool>,
     gateway_peer_id: Arc<AtomicU32>,
 }
 
+#[cfg(any(windows, test))]
 impl ClientRelayStatus {
     const UNKNOWN_GATEWAY_PEER_ID: u32 = 0;
 
@@ -858,6 +851,7 @@ impl ClientRelayStatus {
         }
     }
 
+    #[cfg(windows)]
     fn from_welcome(session: &ClientSession) -> Self {
         let status = Self::new(session.welcome().gateway_connected());
         status.set_gateway_peer_id(session.welcome().gateway_peer_id());
@@ -891,11 +885,13 @@ impl ClientRelayStatus {
     }
 }
 
+#[cfg(any(windows, test))]
 #[derive(Debug, Default)]
 struct ControlEventFormatter {
     peers: BTreeMap<u32, PeerInfo>,
 }
 
+#[cfg(any(windows, test))]
 impl ControlEventFormatter {
     fn format(&mut self, event: &ControlMessage, relay_status: &ClientRelayStatus) -> String {
         match event {
@@ -944,6 +940,7 @@ impl ControlEventFormatter {
     }
 }
 
+#[cfg(any(windows, test))]
 fn format_peer_joined(peer: &PeerInfo) -> String {
     match peer.role() {
         Role::Gateway => {
@@ -1259,11 +1256,6 @@ fn read_and_relay_tap_frame(
     Ok(())
 }
 
-#[cfg(not(windows))]
-fn open_tap_adapter(_session: &ClientSession) {
-    println!("Windows TAP adapter opening is compiled only on Windows");
-}
-
 #[cfg(test)]
 mod tests {
     use std::{