feat(battle-node): WaitingRoom for PvP WS rendezvous

Per-BattleId slot keyed dict. Pair returns the first arriver to the
second; ParkAsync awaits a TCS and returns the second arriver. Timeout
defaults to BattleNodeOptions.WaitingRoomTimeout (60s); evict on timeout
keeps the dict clean. Singleton in DI; consumed by the handler in the
next task.

SVSim.BattleNode/Hosting/BattleNodeExtensions.cs

@@ -30,6 +30,7 @@ public static class BattleNodeExtensions
         services.AddSingleton(options);
         services.AddSingleton<IBattleSessionStore, InMemoryBattleSessionStore>();
         services.AddSingleton<IMatchingBridge, MatchingBridge>();
+        services.AddSingleton<IWaitingRoom, WaitingRoom>();
         services.AddSingleton<BattleNodeWebSocketHandler>();
         return services;
     }