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

README.md

@@ -8,6 +8,7 @@ Monorepo for a Layer 2 over QUIC LAN party bridge.
 - `lanparty-ctrl`: control-plane messages (join/hello/role/version).
 - `lanparty-obs`: shared diagnostics/logging event models.
 - `lanparty-client-core`: platform-agnostic client session state.
+- `lanparty-client-route`: Windows relay-route inspection.
 - `lanparty-client-tap`: TAP-Windows6 adapter discovery and frame I/O.
 - `lanparty-client-win`: Windows TAP + route/metric handling binary.
 - `lanparty-gateway`: Linux AF_PACKET gateway binary.
@@ -48,6 +49,14 @@ Platform-neutral remote client relay session:
 - welcome/reject handling with assigned peer id and effective TAP MTU
 - Ethernet frame send/receive helpers over QUIC DATAGRAM
 
+### `lanparty-client-route`
+
+Windows route-table boundary:
+
+- read-only best-route lookup for a relay destination IP
+- selected source address, next hop, interface index/LUID, prefix, and metric
+- non-Windows builds return a clear unsupported-platform error
+
 ### `lanparty-client-tap`
 
 Windows TAP adapter boundary: