feat(relay): receive peer stats streams

Peers can now send post-handshake Stats messages over unidirectional control
streams. The relay listens for those streams alongside Ethernet datagrams,
stores the latest snapshot on the peer session, and logs the counters with
room, peer, and role context.

Unexpected post-handshake control messages now close the peer as a protocol
error. Peer-requested Disconnect messages leave the room through the normal
cleanup path, which keeps lifecycle notifications centralized.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay \
  forwards_ethernet_datagrams_between_joined_peers \
  -- --nocapture
- cargo test -p lanparty-relay
- cargo clippy -p lanparty-relay --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

README.md

@@ -89,6 +89,7 @@ Public relay binary and relay-owned room state:
 - L2 safety filters for jumbo, switch-control, DHCP-server, and IPv6-RA frames
 - client broadcast/multicast, unknown-unicast, and total bandwidth limiting
 - malformed peer datagram disconnect threshold
+- peer stats control events retained for relay diagnostics
 - peer leave cleanup for room membership and MAC indexes
 
 ## Build