fix(gateway): reject wireless LAN interfaces

The plan explicitly keeps the physical LAN gateway wired-only for the MVP.
Managed Wi-Fi adapters are not reliable for arbitrary source-MAC injection, but
the gateway previously accepted any interface that could be opened as an
Ethernet-like packet socket.

Reject Linux interfaces that sysfs marks as wireless before opening the raw
packet socket. The check looks for the common `wireless` and `phy80211` markers
under `/sys/class/net/<iface>`, and keeps path separators out of interface names
so the sysfs lookup stays scoped to a single netdev name.

Document the wired-only enforcement in the gateway README section.

Test Plan:
- cargo fmt --check
- git diff --check
- cargo test -p lanparty-gateway
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: PLAN.md

crates/lanparty-gateway/src/packet.rs

@@ -2,11 +2,14 @@ use std::{
     ffi::CString,
     io,
     os::fd::{AsRawFd, FromRawFd, OwnedFd, RawFd},
+    path::Path,
 };
 
 use lanparty_proto::MacAddr;
 
 const ETH_P_ALL: u16 = libc::ETH_P_ALL as u16;
+const SYS_CLASS_NET: &str = "/sys/class/net";
+const WIRELESS_INTERFACE_MARKERS: &[&str] = &["wireless", "phy80211"];
 
 #[derive(Debug)]
 pub struct PacketSocket {
@@ -19,6 +22,7 @@ pub struct PacketSocket {
 impl PacketSocket {
     pub fn open(interface: &str) -> io::Result<Self> {
         let interface_index = interface_index(interface)?;
+        reject_wireless_interface(interface)?;
         let protocol = i32::from(ETH_P_ALL.to_be());
         let raw_fd = unsafe {
             // SAFETY: socket is called with constant domain/type/protocol values and returns
@@ -193,6 +197,13 @@ fn interface_name(interface: &str) -> io::Result<CString> {
         ));
     }
 
+    if interface.contains('/') {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidInput,
+            "interface name cannot contain path separators",
+        ));
+    }
+
     let name = CString::new(interface).map_err(|_| {
         io::Error::new(
             io::ErrorKind::InvalidInput,
@@ -209,6 +220,23 @@ fn interface_name(interface: &str) -> io::Result<CString> {
     Ok(name)
 }
 
+fn reject_wireless_interface(interface: &str) -> io::Result<()> {
+    if interface_is_wireless_in_sysfs(Path::new(SYS_CLASS_NET), interface) {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidInput,
+            format!("wireless interface {interface} cannot be used as a LAN gateway"),
+        ));
+    }
+
+    Ok(())
+}
+
+fn interface_is_wireless_in_sysfs(sys_class_net: &Path, interface: &str) -> bool {
+    WIRELESS_INTERFACE_MARKERS
+        .iter()
+        .any(|marker| sys_class_net.join(interface).join(marker).exists())
+}
+
 fn interface_hardware_addr(fd: RawFd, interface: &str) -> io::Result<MacAddr> {
     let name = interface_name(interface)?;
     let mut request = unsafe {
@@ -269,6 +297,11 @@ fn is_inbound_packet_type(packet_type: u8) -> bool {
 
 #[cfg(test)]
 mod tests {
+    use std::{
+        fs,
+        time::{SystemTime, UNIX_EPOCH},
+    };
+
     use super::*;
 
     #[test]
@@ -281,6 +314,10 @@ mod tests {
             interface_index("eth0\0bad").unwrap_err().kind(),
             io::ErrorKind::InvalidInput
         );
+        assert_eq!(
+            interface_index("eth0/bad").unwrap_err().kind(),
+            io::ErrorKind::InvalidInput
+        );
     }
 
     #[test]
@@ -307,4 +344,30 @@ mod tests {
         assert!(is_inbound_packet_type(libc::PACKET_BROADCAST));
         assert!(is_inbound_packet_type(libc::PACKET_MULTICAST));
     }
+
+    #[test]
+    fn detects_wireless_interfaces_from_sysfs_markers() {
+        let sys_class_net = unique_temp_dir("lanparty-gateway-sysfs");
+        let wired = sys_class_net.join("eth0");
+        let wireless = sys_class_net.join("wlan0");
+        let phy_wireless = sys_class_net.join("wlp1s0");
+        fs::create_dir_all(&wired).unwrap();
+        fs::create_dir_all(wireless.join("wireless")).unwrap();
+        fs::create_dir_all(phy_wireless.join("phy80211")).unwrap();
+
+        assert!(!interface_is_wireless_in_sysfs(&sys_class_net, "eth0"));
+        assert!(interface_is_wireless_in_sysfs(&sys_class_net, "wlan0"));
+        assert!(interface_is_wireless_in_sysfs(&sys_class_net, "wlp1s0"));
+
+        fs::remove_dir_all(sys_class_net).unwrap();
+    }
+
+    fn unique_temp_dir(prefix: &str) -> std::path::PathBuf {
+        let nanos = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .as_nanos();
+
+        std::env::temp_dir().join(format!("{prefix}-{}-{nanos}", std::process::id()))
+    }
 }