AF_PACKET sockets can report packets sent by the host as well as packets
received from the LAN. The gateway writes remote-client frames onto the wired
interface, so treating those outgoing packets as fresh LAN input can reflect
self-injected traffic back to the relay.
Read packet metadata with `recvfrom` and skip `PACKET_OUTGOING` frames before
returning a LAN frame to the bridge loop. This keeps capture scoped to inbound
LAN traffic and is a prerequisite for periodic CAM refresh frames.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md gateway AF_PACKET bridge
ddidderr
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