feat(relay): send peer catch-up on join

The relay now sends PeerJoined catch-up events to a newly accepted peer for
peers that were already present in the room. This makes lifecycle delivery
symmetric enough for clients and gateways to learn the current room membership
after welcome, not only future joins.

The catch-up list is built from a cloned room snapshot before opening control
event streams, so room state is not locked across QUIC I/O. Delivery remains
best-effort and uses the same one-frame unidirectional control stream path as
live PeerJoined and PeerLeft notifications.

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

@@ -114,7 +114,8 @@ logged with room, peer, MAC, ethertype, action, drop reason, and target count.
 Unknown unicast from a client is forwarded only to the gateway port; unknown
 unicast from the gateway is dropped instead of flooded to every remote client.
 When a peer joins or leaves, the relay sends a reliable lifecycle control event
-to peers that are still present in the room.
+to peers that are still present in the room. Newly joined peers also receive
+`PeerJoined` events for peers that were already present.
 
 ## Gateway