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

SVSim.BattleNode/Sessions/IBattleSessionStore.cs

@@ -0,0 +1,13 @@
+namespace SVSim.BattleNode.Sessions;
+
+public interface IBattleSessionStore
+{
+    /// <summary>Register a battle minted by the matching bridge, awaiting a WS connect.</summary>
+    void RegisterPending(PendingBattle battle);
+
+    /// <summary>Look up the pending battle. Returns null if not present.</summary>
+    PendingBattle? TryGetPending(string battleId);
+
+    /// <summary>Mark a battle as no longer pending (e.g. on successful connect or explicit close).</summary>
+    bool RemovePending(string battleId);
+}