refactor(engine-ambient): BattleManagerBase.GetIns reads ambient first, static fallback

Step 3 of multi-instancing migration. The dominant per-battle singleton now
resolves through BattleAmbient.Current.Mgr when a scope is active. The legacy
'main' field is renamed _mainFallback and retained for unwrapped callers
(tests, anything not yet scope-wrapped). GetIns() still returns null when
neither is set, preserving the '?.Foo ?? default' patterns in engine code.

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

SVSim.BattleEngine.Tests/BattleAmbientTests.cs

@@ -1,6 +1,7 @@
 #nullable enable
 using SVSim.BattleEngine.Ambient;
 using NUnit.Framework;
+using System.Runtime.Serialization;
 using System.Threading.Tasks;
 
 namespace SVSim.BattleEngine.Tests;
@@ -129,4 +130,22 @@ public class BattleAmbientTests
 
         BattleManagerBase.IsRandomDraw = false;
     }
+
+    [Test]
+    public void GetIns_ReadsAmbient_WhenScopeActive()
+    {
+        var fakeMgr = (BattleManagerBase)System.Runtime.Serialization
+            .FormatterServices.GetUninitializedObject(typeof(BattleManagerBase));
+        var ctx = new BattleAmbientContext { Mgr = fakeMgr };
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(BattleManagerBase.GetIns(), Is.SameAs(fakeMgr));
+    }
+
+    [Test]
+    public void GetIns_OutsideScope_FallsBackToStatic()
+    {
+        Assert.That(BattleAmbient.Current, Is.Null);
+        var v = BattleManagerBase.GetIns();
+        Assert.Pass($"GetIns()={(v is null ? "null" : v.GetType().Name)}");
+    }
 }