feat(battlenode): per-session charaId + single-active-engine gate (Phase 2 N2 carried-risk B)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/Engine/EngineSessionGate.cs

@@ -0,0 +1,17 @@
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// <summary>Process-wide single-active-engine gate. The engine's process-global turn state
+/// (ToolboxGame.RealTimeNetworkAgent, GameMgr) cannot safely back two concurrent battles, so exactly
+/// one BattleSession may own the engine at a time (AskUserQuestion 2026-06-06: serialize + document).
+/// A session that cannot acquire runs WITHOUT the engine and logs it loudly — NOT a silent fallback
+/// (the operator sees the limitation). Per-session isolation (removing this gate) is the tracked
+/// follow-up. Non-blocking TryAcquire keeps it out of the synchronous dispatch path; in local
+/// single-user dev battles are sequential, so contention never arises.</summary>
+internal static class EngineSessionGate
+{
+    private static int _owned; // 0 = free, 1 = owned
+
+    public static bool TryAcquire() => System.Threading.Interlocked.CompareExchange(ref _owned, 1, 0) == 0;
+
+    public static void Release() => System.Threading.Interlocked.Exchange(ref _owned, 0);
+}