feat(client): scope TAP interface MTU while running

The Windows client now sets the TAP IP-interface MTU to the relay-selected MTU
before it starts bridging frames. The override is scoped like the existing
metric and default-route guards, so the previous MTU is restored when the
client exits.

The route crate now exposes `InterfaceMtuSnapshot` and `ScopedInterfaceMtu`
around `MIB_IPINTERFACE_ROW.NlMtu`, reusing the same `GetIpInterfaceEntry` and
`SetIpInterfaceEntry` path already used for metrics and default-route policy.
IPv4 MTU setup is required for startup, while IPv6 MTU setup is best-effort to
match the existing IPv6 route-protection behavior.

This intentionally leaves TAP MAC configuration as fail-fast. TAP-Windows6 does
not expose a matching set-MAC IOCTL in the driver header, so that should remain
a separate design decision.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-route
- cargo test -p lanparty-client-win
- cargo clippy -p lanparty-client-route -p lanparty-client-win --all-targets
  -- -D warnings
- cargo check -p lanparty-client-route --target x86_64-pc-windows-gnu
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- cargo check -p lanparty-client-win --target x86_64-pc-windows-gnu
  (fails before this crate in ring: missing x86_64-w64-mingw32-gcc)

Refs: PLAN.md

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 MTU overrides with restore-on-drop behavior
 - 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
@@ -141,10 +142,10 @@ verifies that the relay route still uses that pinned host route after TAP
 activation, and then bridges Ethernet frames between the relay and the first
 TAP-Windows6 adapter until shutdown.
 `--virtual-mac` can still override the stored identity for manual testing. On
-Windows it marks the TAP media connected and reports the driver MAC/MTU before
-forwarding frames, along with the TAP interface index/LUID. The client applies
-a scoped TAP interface metric and disables TAP default routes while it runs,
-periodically rechecks that the relay route remains pinned, then restores the
-previous route policy on exit. Until automatic TAP MAC/MTU configuration is
-wired, startup fails before bridging if the driver-reported MAC or MTU does not
-match the tunnel settings.
+Windows it sets the TAP IP interface MTU to the relay-selected MTU, marks the
+TAP media connected, and reports the driver MAC/MTU before forwarding frames,
+along with the TAP interface index/LUID. The client applies a scoped TAP
+interface metric and disables TAP default routes while it runs, periodically
+rechecks that the relay route remains pinned, then restores the previous route
+policy on exit. Until automatic TAP MAC configuration is wired, startup fails
+before bridging if the driver-reported MAC does not match the tunnel identity.