feat(client): add scoped relay host route pin

Add the first route-table mutation API to `lanparty-client-route`. Given the
pre-TAP best-route snapshot, the crate can now create a host route for the relay
IP on that same interface and return an RAII guard that deletes the route when
it is dropped.

The route row is initialized with `InitializeIpForwardEntry`, uses a /32 or
/128 destination prefix for the relay IP, preserves the pre-TAP interface
index/LUID, and uses the remembered next hop or an on-link unspecified next hop
when Windows reported no gateway. The guard tracks whether creation actually
succeeded so pure row-construction tests and failed creates do not try to delete
routes they did not install.

This remains a crate-level boundary only. The Windows client still reports the
pre-TAP route but does not yet hold a pin across TAP activation; that is the
next integration slice.

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

Refs: PLAN.md
Refs: https://learn.microsoft.com/en-us/windows-hardware/drivers/network/createipforwardentry2
Refs: https://learn.microsoft.com/en-us/windows-hardware/drivers/network/initializeipforwardentry

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

@@ -1,8 +1,8 @@
 //! Windows route-table inspection for protecting the relay path.
 //!
 //! The client binary uses this crate to keep Win32 route/metric calls out of
-//! the relay session code. This crate starts with read-only route snapshots;
-//! later route pinning can build on the same typed boundary.
+//! the relay session code. The crate can snapshot the current relay route and
+//! install a scoped host route that pins the relay IP to the pre-TAP interface.
 
 use std::net::IpAddr;
 
@@ -91,13 +91,24 @@ impl RouteSnapshot {
 mod windows;
 
 #[cfg(windows)]
-pub use windows::best_route_to;
+pub use windows::{PinnedRelayRoute, best_route_to, pin_relay_route};
 
 #[cfg(not(windows))]
 pub fn best_route_to(_destination: IpAddr) -> Result<RouteSnapshot> {
     bail!("Windows route inspection is only available on Windows");
 }
 
+#[cfg(not(windows))]
+#[derive(Debug)]
+pub struct PinnedRelayRoute {
+    _private: (),
+}
+
+#[cfg(not(windows))]
+pub fn pin_relay_route(_route: &RouteSnapshot) -> Result<PinnedRelayRoute> {
+    bail!("Windows route pinning is only available on Windows");
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -131,6 +142,23 @@ mod tests {
         assert!(best_route_to(ip("203.0.113.10")).is_err());
     }
 
+    #[cfg(not(windows))]
+    #[test]
+    fn rejects_route_pinning_on_non_windows() {
+        let snapshot = RouteSnapshot::new(
+            ip("203.0.113.10"),
+            ip("192.0.2.44"),
+            Some(ip("192.0.2.1")),
+            ip("0.0.0.0"),
+            0,
+            12,
+            34,
+            25,
+        );
+
+        assert!(pin_relay_route(&snapshot).is_err());
+    }
+
     fn ip(value: &str) -> IpAddr {
         value.parse().unwrap()
     }

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

@@ -1,5 +1,5 @@
 use std::{
-    io,
+    fmt, io,
     net::{IpAddr, Ipv4Addr, Ipv6Addr},
     ptr::null,
 };
@@ -7,10 +7,16 @@ use std::{
 use anyhow::{Context, Result};
 use windows_sys::Win32::{
     Foundation::ERROR_SUCCESS,
-    NetworkManagement::IpHelper::{GetBestRoute2, MIB_IPFORWARD_ROW2},
+    NetworkManagement::{
+        IpHelper::{
+            CreateIpForwardEntry2, DeleteIpForwardEntry2, GetBestRoute2, IP_ADDRESS_PREFIX,
+            InitializeIpForwardEntry, MIB_IPFORWARD_ROW2,
+        },
+        Ndis::NET_LUID_LH,
+    },
     Networking::WinSock::{
-        AF_INET, AF_INET6, IN_ADDR, IN_ADDR_0, IN6_ADDR, IN6_ADDR_0, SOCKADDR_IN, SOCKADDR_IN6,
-        SOCKADDR_IN6_0, SOCKADDR_INET,
+        AF_INET, AF_INET6, IN_ADDR, IN_ADDR_0, IN6_ADDR, IN6_ADDR_0, RouteProtocolNetMgmt,
+        SOCKADDR_IN, SOCKADDR_IN6, SOCKADDR_IN6_0, SOCKADDR_INET,
     },
 };
 
@@ -56,6 +62,131 @@ pub fn best_route_to(destination: IpAddr) -> Result<RouteSnapshot> {
     ))
 }
 
+pub fn pin_relay_route(route: &RouteSnapshot) -> Result<PinnedRelayRoute> {
+    let mut pinned = pinned_route_row(route);
+    let status = unsafe {
+        // SAFETY: pinned.row was initialized with InitializeIpForwardEntry and populated with a
+        // valid host destination, next hop, and interface identity.
+        CreateIpForwardEntry2(&pinned.row)
+    };
+    windows_status(status).with_context(|| {
+        format!(
+            "failed to pin relay route to {} via interface index {}",
+            route.destination(),
+            route.interface_index()
+        )
+    })?;
+    pinned.active = true;
+
+    Ok(pinned)
+}
+
+pub struct PinnedRelayRoute {
+    row: MIB_IPFORWARD_ROW2,
+    destination: IpAddr,
+    next_hop: Option<IpAddr>,
+    interface_index: u32,
+    interface_luid: u64,
+    active: bool,
+}
+
+impl PinnedRelayRoute {
+    #[must_use]
+    pub const fn destination(&self) -> IpAddr {
+        self.destination
+    }
+
+    #[must_use]
+    pub const fn next_hop(&self) -> Option<IpAddr> {
+        self.next_hop
+    }
+
+    #[must_use]
+    pub const fn interface_index(&self) -> u32 {
+        self.interface_index
+    }
+
+    #[must_use]
+    pub const fn interface_luid(&self) -> u64 {
+        self.interface_luid
+    }
+}
+
+impl fmt::Debug for PinnedRelayRoute {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("PinnedRelayRoute")
+            .field("destination", &self.destination)
+            .field("next_hop", &self.next_hop)
+            .field("interface_index", &self.interface_index)
+            .field("interface_luid", &self.interface_luid)
+            .field("active", &self.active)
+            .finish_non_exhaustive()
+    }
+}
+
+impl Drop for PinnedRelayRoute {
+    fn drop(&mut self) {
+        if !self.active {
+            return;
+        }
+
+        unsafe {
+            // SAFETY: self.row is the same route row that was successfully created for this guard.
+            // Deletion failure is intentionally ignored during Drop.
+            DeleteIpForwardEntry2(&self.row);
+        }
+    }
+}
+
+fn pinned_route_row(route: &RouteSnapshot) -> PinnedRelayRoute {
+    let destination = route.destination();
+    let next_hop = route.next_hop();
+    let mut row = MIB_IPFORWARD_ROW2::default();
+    unsafe {
+        // SAFETY: row points to valid writable storage for Windows to initialize.
+        InitializeIpForwardEntry(&mut row);
+    }
+    row.InterfaceLuid = NET_LUID_LH {
+        Value: route.interface_luid(),
+    };
+    row.InterfaceIndex = route.interface_index();
+    row.DestinationPrefix = host_prefix(destination);
+    row.NextHop = sockaddr_from_ip(next_hop.unwrap_or_else(|| unspecified_for(destination)));
+    row.SitePrefixLength = host_prefix_len(destination);
+    row.Metric = 0;
+    row.Protocol = RouteProtocolNetMgmt;
+
+    PinnedRelayRoute {
+        row,
+        destination,
+        next_hop,
+        interface_index: route.interface_index(),
+        interface_luid: route.interface_luid(),
+        active: false,
+    }
+}
+
+fn host_prefix(addr: IpAddr) -> IP_ADDRESS_PREFIX {
+    IP_ADDRESS_PREFIX {
+        Prefix: sockaddr_from_ip(addr),
+        PrefixLength: host_prefix_len(addr),
+    }
+}
+
+const fn host_prefix_len(addr: IpAddr) -> u8 {
+    match addr {
+        IpAddr::V4(_) => 32,
+        IpAddr::V6(_) => 128,
+    }
+}
+
+const fn unspecified_for(addr: IpAddr) -> IpAddr {
+    match addr {
+        IpAddr::V4(_) => IpAddr::V4(Ipv4Addr::UNSPECIFIED),
+        IpAddr::V6(_) => IpAddr::V6(Ipv6Addr::UNSPECIFIED),
+    }
+}
+
 fn sockaddr_from_ip(addr: IpAddr) -> SOCKADDR_INET {
     match addr {
         IpAddr::V4(addr) => SOCKADDR_INET {
@@ -125,3 +256,57 @@ fn windows_status(status: u32) -> io::Result<()> {
         Err(io::Error::from_raw_os_error(status as i32))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn builds_ipv4_host_route_row() {
+        let route = RouteSnapshot::new(
+            ip("203.0.113.10"),
+            ip("192.0.2.44"),
+            Some(ip("192.0.2.1")),
+            ip("0.0.0.0"),
+            0,
+            12,
+            34,
+            25,
+        );
+        let pinned = pinned_route_row(&route);
+
+        assert_eq!(pinned.destination(), ip("203.0.113.10"));
+        assert_eq!(pinned.next_hop(), Some(ip("192.0.2.1")));
+        assert_eq!(pinned.interface_index(), 12);
+        assert_eq!(pinned.interface_luid(), 34);
+        assert_eq!(pinned.row.DestinationPrefix.PrefixLength, 32);
+        assert_eq!(pinned.row.SitePrefixLength, 32);
+        assert_eq!(pinned.row.Metric, 0);
+        assert_eq!(pinned.row.Protocol, RouteProtocolNetMgmt);
+    }
+
+    #[test]
+    fn builds_ipv6_on_link_host_route_row() {
+        let route = RouteSnapshot::new(
+            ip("2001:db8::10"),
+            ip("2001:db8::44"),
+            None,
+            ip("2001:db8::"),
+            64,
+            12,
+            34,
+            25,
+        );
+        let pinned = pinned_route_row(&route);
+
+        assert_eq!(pinned.destination(), ip("2001:db8::10"));
+        assert_eq!(pinned.next_hop(), None);
+        assert_eq!(pinned.row.DestinationPrefix.PrefixLength, 128);
+        assert_eq!(pinned.row.SitePrefixLength, 128);
+        assert_eq!(ip_from_sockaddr(&pinned.row.NextHop), Some(ip("::")));
+    }
+
+    fn ip(value: &str) -> IpAddr {
+        value.parse().unwrap()
+    }
+}