Multi-instancing migration (Step 4): convert Wizard.ToolboxGame.RealTimeNetworkAgent — the
per-battle handle to the live PvP/AI network agent (RealTimeNetworkAgent owns the Socket.IO
connection, OnEmit pipeline, and ack bookkeeping for one battle) — to resolve through
BattleAmbient.Current when a scope is active, falling back to a renamed
`_realTimeNetworkAgentFallback` static when not. The original was an auto-property with a
private setter, mutated only via SetRealTimeNetworkBattle / DestroyNetworkAgent; converting
to a manual property + ambient-aware mutators keeps the public surface byte-identical
(design 2026-06-07-engine-multi-instancing, Task 4).

DestroyNetworkAgent preserves the original Unity DestroyImmediate(.gameObject) call exactly,
just reading the "current" agent through the same ambient-first resolution and clearing the
slot it came from. Object.DestroyImmediate resolves via the file's existing `using UnityEngine;`
(no qualifier change needed — matches the file's existing pattern at line 36/79 that calls
`GameObject.Find` unqualified).

In-file references (3 sites) handled as follows:
  - line 28 auto-property         → renamed backing + manual ambient-first getter
  - line 57 SetRealTimeNetworkBattle → writes through Current?.NetworkAgent or fallback
  - line 62 DestroyNetworkAgent   → reads current via ambient-first; clears the same slot

--- Engine/Wizard/ToolboxGame.cs  (~line 28)
-	public static RealTimeNetworkAgent RealTimeNetworkAgent { get; private set; }
+	private static RealTimeNetworkAgent _realTimeNetworkAgentFallback;
+	public static RealTimeNetworkAgent RealTimeNetworkAgent
+	{
+		get => SVSim.BattleEngine.Ambient.BattleAmbient.Current?.NetworkAgent ?? _realTimeNetworkAgentFallback;
+	}

--- Engine/Wizard/ToolboxGame.cs  (~lines 57-69)
 	public static void SetRealTimeNetworkBattle(RealTimeNetworkAgent agent)
 	{
-		RealTimeNetworkAgent = agent;
+		var c = SVSim.BattleEngine.Ambient.BattleAmbient.Current;
+		if (c != null) c.NetworkAgent = agent;
+		else _realTimeNetworkAgentFallback = agent;
 	}

 	public static void DestroyNetworkAgent()
 	{
-		if (RealTimeNetworkAgent != null)
+		var c = SVSim.BattleEngine.Ambient.BattleAmbient.Current;
+		var current = c?.NetworkAgent ?? _realTimeNetworkAgentFallback;
+		if (current != null)
 		{
-			Object.DestroyImmediate(RealTimeNetworkAgent.gameObject);
-			RealTimeNetworkAgent = null;
+			Object.DestroyImmediate(current.gameObject);
+			if (c != null) c.NetworkAgent = null;
+			else _realTimeNetworkAgentFallback = null;
 		}
 	}