feat(client): receive relay control events

ClientSession can now accept one-frame relay control events sent on reliable
unidirectional QUIC streams. This gives the Windows client a core API for the
PeerJoined and PeerLeft lifecycle messages that the relay now emits.

The implementation stays in client-core because it shares the relay connection
and control codec with the handshake. Client UI/status handling remains a
separate slice so this commit only establishes the tested transport boundary.

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

Refs: PLAN.md

README.md

@@ -49,6 +49,7 @@ Platform-neutral remote client relay session:
 - client hello with room, virtual MAC, and datagram budget
 - welcome/reject handling with assigned peer id and effective TAP MTU
 - QUIC DATAGRAM support and negotiated datagram budget diagnostics
+- reliable relay control-event reads for peer lifecycle messages
 - Ethernet frame send/receive helpers over QUIC DATAGRAM
 - client tunnel statistics for frame/datagram rx/tx and drops