feat(client): add scoped interface metric override

Add a reversible IP interface metric boundary to `lanparty-client-route`. The
crate can now read an IPv4 or IPv6 interface metric snapshot and temporarily set
a manual interface metric with an RAII guard that restores the previous metric
and automatic-metric state on drop.

This prepares the TAP metric handling without wiring policy into the Windows
client yet. Default-route disabling is captured in snapshots for diagnostics and
future decisions, but this slice deliberately changes only `UseAutomaticMetric`
and `Metric`.

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
Refs: https://learn.microsoft.com/en-us/windows-hardware/drivers/network/initializeipinterfaceentry
Refs: https://learn.microsoft.com/en-us/windows/win32/api/netioapi/nf-netioapi-setipinterfaceentry

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

@@ -44,6 +44,65 @@ impl NetworkInterfaceIdentity {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum IpInterfaceFamily {
+    Ipv4,
+    Ipv6,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct InterfaceMetricSnapshot {
+    identity: NetworkInterfaceIdentity,
+    family: IpInterfaceFamily,
+    automatic_metric: bool,
+    metric: u32,
+    disable_default_routes: bool,
+}
+
+impl InterfaceMetricSnapshot {
+    #[cfg_attr(not(windows), allow(dead_code))]
+    const fn new(
+        identity: NetworkInterfaceIdentity,
+        family: IpInterfaceFamily,
+        automatic_metric: bool,
+        metric: u32,
+        disable_default_routes: bool,
+    ) -> Self {
+        Self {
+            identity,
+            family,
+            automatic_metric,
+            metric,
+            disable_default_routes,
+        }
+    }
+
+    #[must_use]
+    pub const fn identity(self) -> NetworkInterfaceIdentity {
+        self.identity
+    }
+
+    #[must_use]
+    pub const fn family(self) -> IpInterfaceFamily {
+        self.family
+    }
+
+    #[must_use]
+    pub const fn automatic_metric(self) -> bool {
+        self.automatic_metric
+    }
+
+    #[must_use]
+    pub const fn metric(self) -> u32 {
+        self.metric
+    }
+
+    #[must_use]
+    pub const fn disable_default_routes(self) -> bool {
+        self.disable_default_routes
+    }
+}
+
 impl RouteSnapshot {
     #[cfg_attr(not(windows), allow(dead_code))]
     #[allow(clippy::too_many_arguments)]
@@ -115,6 +174,8 @@ mod windows;
 
 #[cfg(windows)]
 pub use windows::{PinnedRelayRoute, best_route_to, interface_identity_from_guid, pin_relay_route};
+#[cfg(windows)]
+pub use windows::{ScopedInterfaceMetric, interface_metric, set_scoped_interface_metric};
 
 #[cfg(not(windows))]
 pub fn best_route_to(_destination: IpAddr) -> Result<RouteSnapshot> {
@@ -137,6 +198,29 @@ pub fn interface_identity_from_guid(_interface_guid: &str) -> Result<NetworkInte
     bail!("Windows interface identity lookup is only available on Windows");
 }
 
+#[cfg(not(windows))]
+#[derive(Debug)]
+pub struct ScopedInterfaceMetric {
+    _private: (),
+}
+
+#[cfg(not(windows))]
+pub fn interface_metric(
+    _identity: NetworkInterfaceIdentity,
+    _family: IpInterfaceFamily,
+) -> Result<InterfaceMetricSnapshot> {
+    bail!("Windows interface metric lookup is only available on Windows");
+}
+
+#[cfg(not(windows))]
+pub fn set_scoped_interface_metric(
+    _identity: NetworkInterfaceIdentity,
+    _family: IpInterfaceFamily,
+    _metric: u32,
+) -> Result<ScopedInterfaceMetric> {
+    bail!("Windows interface metric updates are only available on Windows");
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -172,6 +256,19 @@ mod tests {
         assert_eq!(identity.luid(), 34);
     }
 
+    #[test]
+    fn exposes_interface_metric_snapshot_fields() {
+        let identity = NetworkInterfaceIdentity::new(12, 34);
+        let snapshot =
+            InterfaceMetricSnapshot::new(identity, IpInterfaceFamily::Ipv4, true, 25, false);
+
+        assert_eq!(snapshot.identity(), identity);
+        assert_eq!(snapshot.family(), IpInterfaceFamily::Ipv4);
+        assert!(snapshot.automatic_metric());
+        assert_eq!(snapshot.metric(), 25);
+        assert!(!snapshot.disable_default_routes());
+    }
+
     #[cfg(not(windows))]
     #[test]
     fn rejects_route_inspection_on_non_windows() {
@@ -201,6 +298,15 @@ mod tests {
         assert!(interface_identity_from_guid("{00112233-4455-6677-8899-AABBCCDDEEFF}").is_err());
     }
 
+    #[cfg(not(windows))]
+    #[test]
+    fn rejects_interface_metric_operations_on_non_windows() {
+        let identity = NetworkInterfaceIdentity::new(12, 34);
+
+        assert!(interface_metric(identity, IpInterfaceFamily::Ipv4).is_err());
+        assert!(set_scoped_interface_metric(identity, IpInterfaceFamily::Ipv4, 500).is_err());
+    }
+
     fn ip(value: &str) -> IpAddr {
         value.parse().unwrap()
     }