refactor(engine-ambient): ViewerId/RealTimeNetworkAgent/BattleRecoveryInfo read ambient first

Step 4 of multi-instancing migration. Three additional per-battle statics
front-fronted by BattleAmbient.Current, each with a static fallback for
unwrapped callers. ViewerId's SavedataManager-persisting setter is preserved
on the fallback path; inside a scope, the setter is a no-op (the per-battle
perspective is fixed at scope entry).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SVSim.BattleEngine.Tests/BattleAmbientTests.cs

@@ -148,4 +148,45 @@ public class BattleAmbientTests
         var v = BattleManagerBase.GetIns();
         Assert.Pass($"GetIns()={(v is null ? "null" : v.GetType().Name)}");
     }
+
+    [Test]
+    public void ViewerId_ReadsAmbient_WhenScopeActive()
+    {
+        var ctx = new BattleAmbientContext { ViewerId = 12345 };
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(Cute.Certification.ViewerId, Is.EqualTo(12345));
+    }
+
+    [Test]
+    public void RealTimeNetworkAgent_ReadsAmbient_WhenScopeActive()
+    {
+        var ctx = new BattleAmbientContext();
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(Wizard.ToolboxGame.RealTimeNetworkAgent, Is.Null);
+        var agent = (RealTimeNetworkAgent)System.Runtime.Serialization
+            .FormatterServices.GetUninitializedObject(typeof(RealTimeNetworkAgent));
+        ctx.NetworkAgent = agent;
+        Assert.That(Wizard.ToolboxGame.RealTimeNetworkAgent, Is.SameAs(agent));
+    }
+
+    [Test]
+    public void SetRealTimeNetworkBattle_InsideScope_WritesAmbient()
+    {
+        var ctx = new BattleAmbientContext();
+        using var _ = BattleAmbient.Enter(ctx);
+        var agent = (RealTimeNetworkAgent)System.Runtime.Serialization
+            .FormatterServices.GetUninitializedObject(typeof(RealTimeNetworkAgent));
+        Wizard.ToolboxGame.SetRealTimeNetworkBattle(agent);
+        Assert.That(ctx.NetworkAgent, Is.SameAs(agent));
+    }
+
+    [Test]
+    public void BattleRecoveryInfo_ReadsAmbient_WhenScopeActive()
+    {
+        var info = (Wizard.BattleRecoveryInfo)System.Runtime.Serialization
+            .FormatterServices.GetUninitializedObject(typeof(Wizard.BattleRecoveryInfo));
+        var ctx = new BattleAmbientContext { RecoveryInfo = info };
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(Wizard.Data.BattleRecoveryInfo, Is.SameAs(info));
+    }
 }