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/Bridge/BattleNodeOptions.cs

@@ -8,4 +8,11 @@ namespace SVSim.BattleNode.Bridge;
 public sealed class BattleNodeOptions
 {
     public string NodeServerUrl { get; set; } = "localhost:5148/socket.io/";
+
+    /// <summary>
+    /// How long the first arriver's WS waits for a partner before disconnecting.
+    /// Matches the architecture spec's 60s default; override (typically lower)
+    /// in tests via the factory.
+    /// </summary>
+    public TimeSpan WaitingRoomTimeout { get; set; } = TimeSpan.FromSeconds(60);
 }