fix(client): fail fast outside Windows

The MVP client only has a real TAP and route-protection runtime on Windows. A
non-Windows build was able to connect to the relay as a no-TAP client and then
wait for control events, which could consume a room slot while giving the false
impression that the Windows client path had been exercised.

Reject normal runtime startup on non-Windows platforms before reading tunnel
inputs or joining the relay. Keep non-Windows builds useful for type checking,
and document that they are not runnable tunnel clients.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-win
- cargo check -p lanparty-client-win
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo run -p lanparty-client-win -- --relay 127.0.0.1:1 \
  --relay-ca-cert /tmp/does-not-matter.der --room ROOM1
- git diff --check
- git diff --cached --check

Refs: MVP Windows TAP client boundary

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

@@ -172,6 +172,7 @@ async fn main() -> Result<()> {
         return Ok(());
     }
 
+    ensure_supported_platform()?;
     let startup_config = args.into_startup_config()?;
     #[cfg(windows)]
     prepare_tap_before_relay_connect(
@@ -218,6 +219,18 @@ async fn main() -> Result<()> {
     run_result
 }
 
+#[cfg(windows)]
+fn ensure_supported_platform() -> Result<()> {
+    Ok(())
+}
+
+#[cfg(not(windows))]
+fn ensure_supported_platform() -> Result<()> {
+    bail!(
+        "lanparty-client-win can only run on Windows; TAP-Windows6 and relay route protection are Windows-only"
+    )
+}
+
 fn print_available_tap_adapters() -> Result<()> {
     let adapters = lanparty_client_tap::available_adapters()?;
     for line in format_tap_adapter_list(&adapters) {
@@ -1235,6 +1248,20 @@ mod tests {
     use lanparty_net::DEFAULT_RELAY_PORT;
     use lanparty_obs::{QuicDiagnostics, TunnelStats};
 
+    #[cfg(not(windows))]
+    #[test]
+    fn rejects_runtime_on_non_windows() {
+        let error = ensure_supported_platform().unwrap_err();
+
+        assert!(error.to_string().contains("can only run on Windows"));
+    }
+
+    #[cfg(windows)]
+    #[test]
+    fn accepts_runtime_on_windows() {
+        assert!(ensure_supported_platform().is_ok());
+    }
+
     #[test]
     fn accepts_matching_tap_driver_mac() {
         assert!(validate_tap_driver_mac(mac(1), mac(1)).is_ok());