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PLAN.md requires rejecting broadcast, multicast, and otherwise invalid source MACs. Client ingress already had a forged-source check against the registered client identity, but gateway ingress could still forward Ethernet frames whose source MAC was not valid unicast. Add an explicit `InvalidSourceMac` drop reason and filter invalid Ethernet source MACs before client source authorization and last-seen refresh. This keeps invalid source addresses out of both remote-client and gateway forwarding paths while preserving `UnauthorizedSourceMac` for valid unicast sources that simply belong to another client identity. Document the invalid-source filter in the relay README decomposition. Test Plan: - cargo fmt --check - cargo test -p lanparty-obs -p lanparty-relay - cargo test --workspace - cargo clippy --workspace --all-targets -- -D warnings - git diff --check Refs: PLAN.md