The relay already filters invalid source MACs, jumbo frames, and L2 control
plane traffic. The gateway bridge was still encoding those LAN frames and
sending them to the relay first, so gateway logs could say forwarded even when
the relay would later drop the frame.
Classify that same local LAN-send subset before QUIC DATAGRAM encoding. The
gateway now records and reports these frames as local drops, keeps the relay as
the trust boundary, and avoids spending relay bandwidth on frames that can never
reach remote clients.
Document that gateway-side local drops cover invalid source MACs, L2 control
plane traffic, jumbo frames, and datagram-budget failures.
Test Plan:
- cargo test -p lanparty-gateway
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md LAN-to-remote control-plane filtering
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