feat(gateway): refresh remote MAC learning

The LAN switch has to keep learning that remote client MAC addresses live on
the gateway port. Once the gateway injects a remote client's traffic, that
learning can age out if the client is quiet, breaking the Layer 2 illusion.

Track valid remote-client source MACs observed in relay traffic and inject a
small padded CAM refresh frame for each known MAC every 60 seconds. The refresh
frame uses the remote MAC as the Ethernet source and the gateway NIC MAC as the
destination, with a local experimental EtherType so hosts should ignore it.

PacketSocket now reads the wired interface hardware address with SIOCGIFHWADDR
when opening the AF_PACKET socket. Non-Ethernet or invalid source interfaces
fail early instead of starting a gateway that cannot emit refresh traffic.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md Linux gateway CAM-table refresh

README.md

@@ -93,7 +93,9 @@ cargo run -p lanparty-gateway -- \
 The gateway connects to the relay as `role = gateway`, completes the
 control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
 interface, and bridges Ethernet frames between the relay and wired LAN until
-shutdown.
+shutdown. It tracks remote-client source MACs seen from relay traffic and
+periodically emits small CAM refresh frames so the physical switch keeps those
+MACs associated with the gateway port.
 
 ## Windows Client