feat(client): resolve Windows interface identity by GUID

Add a small route-crate API that resolves a Windows network adapter GUID into
its interface LUID and index. The TAP adapter discovery already gives us the
NetCfgInstanceId GUID, and the route/metric work needs the corresponding IP
interface identity before it can safely inspect or adjust TAP metrics.

The implementation keeps GUID parsing local and dependency-free, then delegates
the actual identity lookup to Windows IP Helper calls. Non-Windows builds expose
the same API shape with a clear unsupported-platform error.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo check -p lanparty-client-route --target x86_64-pc-windows-msvc
- cargo clippy -p lanparty-client-route --target x86_64-pc-windows-msvc --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

crates/lanparty-client-route/src/lib.rs

@@ -21,6 +21,29 @@ pub struct RouteSnapshot {
     metric: u32,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct NetworkInterfaceIdentity {
+    index: u32,
+    luid: u64,
+}
+
+impl NetworkInterfaceIdentity {
+    #[cfg_attr(not(windows), allow(dead_code))]
+    const fn new(index: u32, luid: u64) -> Self {
+        Self { index, luid }
+    }
+
+    #[must_use]
+    pub const fn index(self) -> u32 {
+        self.index
+    }
+
+    #[must_use]
+    pub const fn luid(self) -> u64 {
+        self.luid
+    }
+}
+
 impl RouteSnapshot {
     #[cfg_attr(not(windows), allow(dead_code))]
     #[allow(clippy::too_many_arguments)]
@@ -91,7 +114,7 @@ impl RouteSnapshot {
 mod windows;
 
 #[cfg(windows)]
-pub use windows::{PinnedRelayRoute, best_route_to, pin_relay_route};
+pub use windows::{PinnedRelayRoute, best_route_to, interface_identity_from_guid, pin_relay_route};
 
 #[cfg(not(windows))]
 pub fn best_route_to(_destination: IpAddr) -> Result<RouteSnapshot> {
@@ -109,6 +132,11 @@ pub fn pin_relay_route(_route: &RouteSnapshot) -> Result<PinnedRelayRoute> {
     bail!("Windows route pinning is only available on Windows");
 }
 
+#[cfg(not(windows))]
+pub fn interface_identity_from_guid(_interface_guid: &str) -> Result<NetworkInterfaceIdentity> {
+    bail!("Windows interface identity lookup is only available on Windows");
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -136,6 +164,14 @@ mod tests {
         assert_eq!(snapshot.metric(), 25);
     }
 
+    #[test]
+    fn exposes_network_interface_identity_fields() {
+        let identity = NetworkInterfaceIdentity::new(12, 34);
+
+        assert_eq!(identity.index(), 12);
+        assert_eq!(identity.luid(), 34);
+    }
+
     #[cfg(not(windows))]
     #[test]
     fn rejects_route_inspection_on_non_windows() {
@@ -159,6 +195,12 @@ mod tests {
         assert!(pin_relay_route(&snapshot).is_err());
     }
 
+    #[cfg(not(windows))]
+    #[test]
+    fn rejects_interface_lookup_on_non_windows() {
+        assert!(interface_identity_from_guid("{00112233-4455-6677-8899-AABBCCDDEEFF}").is_err());
+    }
+
     fn ip(value: &str) -> IpAddr {
         value.parse().unwrap()
     }