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.UnitTests/BattleNode/Hosting/WaitingRoomTests.cs

@@ -0,0 +1,78 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NUnit.Framework;
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Hosting;
+using SVSim.BattleNode.Sessions.Participants;
+using SVSim.UnitTests.BattleNode.Infrastructure;
+
+namespace SVSim.UnitTests.BattleNode.Hosting;
+
+[TestFixture]
+public class WaitingRoomTests
+{
+    [Test]
+    public void Pair_on_empty_slot_returns_null()
+    {
+        var room = new WaitingRoom();
+        var participant = NewParticipant(viewerId: 1);
+
+        var paired = room.Pair("bid-1", participant);
+
+        Assert.That(paired, Is.Null);
+    }
+
+    [Test]
+    public async Task Park_then_Pair_resolves_with_each_arriver_seeing_the_other()
+    {
+        var room = new WaitingRoom();
+        var first = NewParticipant(viewerId: 1);
+        var second = NewParticipant(viewerId: 2);
+
+        var parkTask = room.ParkAsync("bid-1", first, TimeSpan.FromSeconds(5), CancellationToken.None);
+        // Yield so Park's TryAdd lands first.
+        await Task.Yield();
+
+        var firstReturnedToSecond = room.Pair("bid-1", second);
+        var secondReturnedToFirst = await parkTask;
+
+        Assert.That(firstReturnedToSecond, Is.SameAs(first),
+            "Pair must return the first arriver to the second.");
+        Assert.That(secondReturnedToFirst, Is.SameAs(second),
+            "Park must return the second arriver to the first.");
+    }
+
+    [Test]
+    public async Task Park_times_out_returns_null_and_evicts_slot()
+    {
+        var room = new WaitingRoom();
+        var first = NewParticipant(viewerId: 1);
+
+        var second = await room.ParkAsync("bid-1", first, TimeSpan.FromMilliseconds(50), CancellationToken.None);
+
+        Assert.That(second, Is.Null);
+        // Slot should be evicted; a subsequent Pair returns null (no first arriver).
+        var paired = room.Pair("bid-1", NewParticipant(viewerId: 2));
+        Assert.That(paired, Is.Null);
+    }
+
+    [Test]
+    public void Evict_is_idempotent()
+    {
+        var room = new WaitingRoom();
+
+        Assert.DoesNotThrow(() => room.Evict("bid-1"));
+        Assert.DoesNotThrow(() => room.Evict("bid-1"));
+    }
+
+    private static RealParticipant NewParticipant(long viewerId)
+    {
+        var ws = new TestWebSocket();
+        var ctx = new MatchContext(
+            SelfDeckCardIds: Array.Empty<long>(),
+            ClassId: "1", CharaId: "1", CardMasterName: "card_master_node_10015",
+            CountryCode: "KOR", UserName: "Player", SleeveId: "0",
+            EmblemId: "0", DegreeId: "0", FieldId: 0, IsOfficial: 0,
+            BattleType: 11);
+        return new RealParticipant(ws, viewerId, ctx, NullLogger<RealParticipant>.Instance);
+    }
+}