feat(client): add scoped default-route suppression

The Windows route helper now supports a scoped DisableDefaultRoutes override
for an IP interface family. The guard snapshots the existing interface row,
sets the requested default-route disabled state, and restores the previous
state when dropped.

This is the route-crate half of neutralizing TAP default-route takeover from
PLAN.md. It intentionally does not wire the client yet, so the public API can
be reviewed independently from the client startup behavior.

The implementation keeps metric restoration and default-route restoration
separate even though both read the same IP interface row. That avoids one guard
accidentally reverting the other guard's setting when the Windows client holds
both at the same time.

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
- git diff --check

Refs: PLAN.md

README.md

@@ -57,6 +57,7 @@ Windows route-table boundary:
 - selected source address, next hop, interface index/LUID, prefix, and metric
 - interface index/LUID lookup from Windows network adapter GUIDs
 - scoped IP interface metric overrides with restore-on-drop behavior
+- scoped default-route suppression with restore-on-drop behavior
 - scoped host-route pinning for the relay IP on the pre-TAP interface
 - non-Windows builds return a clear unsupported-platform error