PLAN.md calls for a user-facing warning when TAP routing could steal the
relay path. The client already disables TAP default routes under a scoped guard,
but the startup output only reported the previous raw flag value.
Format the route-protection message as a warning whenever default routes were
enabled before the scoped override. Keep the already-disabled case quiet and
explicit, and cover both messages with tests that run on non-Windows builds.
Document the startup warning alongside the existing route-protection behavior.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-win -p lanparty-client-route
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md
ddidderr
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