feat(battle-node): in-memory IBattleSessionStore + PendingBattle

SVSim.BattleNode/Sessions/InMemoryBattleSessionStore.cs

@@ -0,0 +1,17 @@
+using System.Collections.Concurrent;
+
+namespace SVSim.BattleNode.Sessions;
+
+public sealed class InMemoryBattleSessionStore : IBattleSessionStore
+{
+    private readonly ConcurrentDictionary<string, PendingBattle> _pending = new();
+
+    public void RegisterPending(PendingBattle battle) =>
+        _pending[battle.BattleId] = battle;
+
+    public PendingBattle? TryGetPending(string battleId) =>
+        _pending.TryGetValue(battleId, out var b) ? b : null;
+
+    public bool RemovePending(string battleId) =>
+        _pending.TryRemove(battleId, out _);
+}