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
