feat(client): add scoped interface metric override

Add a reversible IP interface metric boundary to `lanparty-client-route`. The
crate can now read an IPv4 or IPv6 interface metric snapshot and temporarily set
a manual interface metric with an RAII guard that restores the previous metric
and automatic-metric state on drop.

This prepares the TAP metric handling without wiring policy into the Windows
client yet. Default-route disabling is captured in snapshots for diagnostics and
future decisions, but this slice deliberately changes only `UseAutomaticMetric`
and `Metric`.

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
Refs: https://learn.microsoft.com/en-us/windows-hardware/drivers/network/initializeipinterfaceentry
Refs: https://learn.microsoft.com/en-us/windows/win32/api/netioapi/nf-netioapi-setipinterfaceentry

README.md

@@ -56,6 +56,7 @@ 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 IP interface metric overrides 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