feat(client): resolve Windows interface identity by GUID

Add a small route-crate API that resolves a Windows network adapter GUID into
its interface LUID and index. The TAP adapter discovery already gives us the
NetCfgInstanceId GUID, and the route/metric work needs the corresponding IP
interface identity before it can safely inspect or adjust TAP metrics.

The implementation keeps GUID parsing local and dependency-free, then delegates
the actual identity lookup to Windows IP Helper calls. Non-Windows builds expose
the same API shape with a clear unsupported-platform error.

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

README.md

@@ -55,6 +55,7 @@ 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
+- interface index/LUID lookup from Windows network adapter GUIDs
 - scoped host-route pinning for the relay IP on the pre-TAP interface
 - non-Windows builds return a clear unsupported-platform error