LAN-to-remote switch-control filtering is enforced by the gateway and relay,
but the Windows client is the final boundary before frames enter the TAP
adapter. A malformed or buggy relay path should not be able to make the client
write LAN control traffic, invalid-source frames, or jumbo frames into TAP.
Reuse the shared gateway/LAN safety classifier on received relay Ethernet
frames. Filtered frames are counted and skipped, and recv_ethernet only returns
frames that are safe to hand to the platform TAP writer.
Extend the client relay-session test so the mock relay sends a filtered frame
before the valid one, then document the receive-side TAP boundary in the
README.
Test Plan:
- cargo test -p lanparty-client-core
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo fmt --check
- git diff --check
Refs: PLAN.md LAN-to-remote control-plane filtering
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