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/IWaitingRoom.cs

@@ -0,0 +1,26 @@
+using SVSim.BattleNode.Sessions.Participants;
+
+namespace SVSim.BattleNode.Hosting;
+
+/// <summary>
+/// Per-BattleId WS rendezvous for PvP. First arriver parks; second arriver pairs.
+/// The handler reads the result and either constructs the session (second arriver)
+/// or awaits termination via the participant's session-finished signal (first arriver).
+/// </summary>
+public interface IWaitingRoom
+{
+    /// <summary>Try to claim a previously-parked first arriver. Returns the first
+    /// arriver (and clears the slot) if one is parked; null if this caller is the
+    /// first arriver (caller should then ParkAsync).</summary>
+    RealParticipant? Pair(string battleId, RealParticipant self);
+
+    /// <summary>Park as the first arriver; await pairing or timeout. Returns the
+    /// second arriver on pairing; null on timeout / cancellation / TryAdd race.</summary>
+    Task<RealParticipant?> ParkAsync(string battleId, RealParticipant self,
+        TimeSpan timeout, CancellationToken ct);
+
+    /// <summary>Best-effort cleanup; idempotent. Called on timeout or cancellation
+    /// so a stale TCS doesn't linger if the first arriver disconnects before
+    /// pairing.</summary>
+    void Evict(string battleId);
+}