feat(client): add Windows route snapshot helper

Add `lanparty-client-route` as the Win32 boundary for route-table work. The
first API is intentionally read-only: `best_route_to` wraps `GetBestRoute2`
and returns the selected source address, next hop, route prefix, interface
index/LUID, and route metric for a relay destination IP.

This keeps the route-protection work separate from the QUIC client binary, so
we can typecheck the Windows IP Helper calls on this Linux host without pulling
in the `ring` build path that currently blocks full Windows binary checks.
Actual route pinning and metric mutation remain later slices.

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 -- -D warnings
- git diff --check

Refs: PLAN.md

crates/lanparty-client-route/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "lanparty-client-route"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+anyhow.workspace = true
+
+[target.'cfg(windows)'.dependencies]
+windows-sys = { workspace = true, features = [
+  "Win32_Foundation",
+  "Win32_NetworkManagement_IpHelper",
+  "Win32_NetworkManagement_Ndis",
+  "Win32_Networking_WinSock",
+] }