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
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