feat(relay): close graceful disconnect peers

Peers can now end their relay session with a post-handshake Disconnect
control message, but the relay previously only returned from the peer loop.
Explicitly close that peer connection before leaving the room so graceful
disconnects are finite from both sides.

The relay still runs the normal leave path after the close. That keeps room
cleanup, session removal, and PeerLeft notification centralized, while the
PeerLeft reason now comes from the peer's Disconnect message.

The integration test covers two connected clients. One sends a TimedOut
Disconnect, the relay closes that client connection, and the remaining peer
receives PeerLeft with the TimedOut reason.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay \
  forwards_graceful_disconnect_reason_to_remaining_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

@@ -92,6 +92,7 @@ Public relay binary and relay-owned room state:
 - client broadcast/multicast, unknown-unicast, and total bandwidth limiting
 - malformed peer datagram disconnect threshold
 - peer stats control events retained for relay diagnostics
+- graceful disconnect control events propagated as peer-leave reasons
 - peer leave cleanup for room membership and MAC indexes
 
 ## Build