feat(client): log relay lifecycle events

The Windows client now listens for relay control events while the TAP frame pump
is running and logs peer lifecycle updates as they arrive. Gateway joins get a
clear LAN-gateway message, client joins include their virtual MAC, and peer
leaves include the relay-provided reason.

The non-Windows placeholder path also listens for the same events while waiting
for Ctrl-C. That keeps lifecycle diagnostics visible in local relay/client smoke
tests even before the Windows TAP path can be exercised on a real machine.

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

Refs: PLAN.md

README.md

@@ -165,3 +165,5 @@ policy on exit. Until automatic TAP MAC configuration is wired, startup fails
 before bridging if the driver-reported MAC does not match the tunnel identity.
 It prints client diagnostics snapshots with relay reachability, route-pinning,
 QUIC datagram budget, TAP status/IP, frame/datagram counters, and drops.
+Relay lifecycle events are logged as they arrive, including gateway joins and
+peer leaves.