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)}");
+    }
 }

SVSim.BattleEngine/COPIED.manifest.tsv

@@ -62,7 +62,7 @@ BattleFinishToOpponentDisConnectChecker.cs	BattleFinishToOpponentDisConnectCheck
 BattleKeywordInfoListMgr.cs	BattleKeywordInfoListMgr.cs	a014170d0b3f5499635bcc2e29755dc2f3125d5a5a28b1741a4abc74b4abcf86	0
 BattleLifeTimeSharedObject.cs	BattleLifeTimeSharedObject.cs	ab8bc3703d268752a1de56ab5d3e9ebd276980c20076eb0ca300838b3db13d5f	0
 BattleLogTextBuilderAttachSkill.cs	BattleLogTextBuilderAttachSkill.cs	11c585ae931fa3dc734bb231d6da61df3b51b803516ca2c5d88a0c78bc7c0104	0
-BattleManagerBase.cs	BattleManagerBase.cs	6e537b57a66d16c056f62ed3d00c1727d08adab098e90b4f1373c987a2da8a35	1
+BattleManagerBase.cs	BattleManagerBase.cs	d12aed345f596e068bfb130a247e280457cf5160a76f8bfef4a1dda1c561d196	1
 BattleMenuMgr.cs	BattleMenuMgr.cs	7418699063e01641d0df1ed16773a9ac9418f418cc047fc18c5892eb7971d361	0
 BattlePlayer.cs	BattlePlayer.cs	001409844b46ddaf0a5edbce4e015749ece61053adf725a978987d7063a02632	0
 BattlePlayerBase.cs	BattlePlayerBase.cs	9d3a665158706460a52900008dcfcdf575dbe08cb6d3cc05e63e718b2885b51b	0

SVSim.BattleEngine/Engine/BattleManagerBase.cs

@@ -410,7 +410,7 @@ public class BattleManagerBase
 
 	private NetworkTouchControl _networkTouchControl;
 
-	private static BattleManagerBase main;
+	private static BattleManagerBase _mainFallback;
 
 	private static bool _isRandomDrawFallback = false;
 	public static bool IsRandomDraw {
@@ -724,12 +724,12 @@ public class BattleManagerBase
 
 	public static BattleManagerBase GetIns()
 	{
-		return main;
+		return SVSim.BattleEngine.Ambient.BattleAmbient.Current?.Mgr ?? _mainFallback;
 	}
 
 	protected BattleManagerBase(IBattleMgrContentsCreator contentsCreator)
 	{
-		main = this;
+		_mainFallback = this;
 		BattleLifeTimeSharedObject = new BattleLifeTimeSharedObject();
 		PublishedSkillList = new List<SkillBase>();
 		_contentsCreator = contentsCreator;
@@ -1488,7 +1488,7 @@ public class BattleManagerBase
 		GameMgr.GetIns().GetPrefabMgr().DisposeAllClonedObject();
 		GameMgr.GetIns().GetGameObjMgr().DisposeUIGameObj();
 		GameMgr.GetIns().GetPrefabMgr().AllUnLoad();
-		main = null;
+		_mainFallback = null;
 	}
 
 	private void DisposeBattleGameObj_DestroyImmediate(GameObject obj)

SVSim.BattleEngine/Patches/BattleManagerBase.ambient-main-getins.patch

@@ -0,0 +1,40 @@
+Multi-instancing migration (Step 3): convert the dominant per-battle singleton seam — the
+static `main` backing field and the `GetIns()` accessor — to read through BattleAmbient.Current
+first, falling back to a renamed `_mainFallback` static when no scope is active. The legacy
+field is kept (renamed) so unwrapped callers (SingleBattleMgr today, in-process tests that
+don't yet wrap in BattleAmbient.Enter) keep working with zero behavioral change; scoped
+callers get per-AsyncLocal isolation so sibling battles in the same process don't clobber
+each other's "current manager" (design 2026-06-07-engine-multi-instancing, Task 3).
+
+GetIns() is the canonical accessor used by ~hundreds of engine call sites; routing it through
+ambient is what makes the rest of the migration value-adding. Direct cross-file reads of
+`BattleManagerBase.main` were already absent (the field was private; only an in-comment
+reference in Shim/BattleAmbientContext.cs exists), so no external callers needed updating.
+
+In-file references (4 total) handled as follows:
+  - line 413 declaration         → renamed to `_mainFallback`
+  - line 725 GetIns body          → ambient-first via Current?.Mgr ?? _mainFallback
+  - line 730 ctor `main = this`   → `_mainFallback = this`
+  - line 1491 dispose `main=null` → `_mainFallback = null`
+
+--- Engine/BattleManagerBase.cs  (~line 413)
+-	private static BattleManagerBase main;
++	private static BattleManagerBase _mainFallback;
+
+--- Engine/BattleManagerBase.cs  (~line 725)
+ 	public static BattleManagerBase GetIns()
+ 	{
+-		return main;
++		return SVSim.BattleEngine.Ambient.BattleAmbient.Current?.Mgr ?? _mainFallback;
+ 	}
+
+--- Engine/BattleManagerBase.cs  (~line 730)
+ 	protected BattleManagerBase(IBattleMgrContentsCreator contentsCreator)
+ 	{
+-		main = this;
++		_mainFallback = this;
+
+--- Engine/BattleManagerBase.cs  (~line 1491)
+-		main = null;
++		_mainFallback = null;
+ 	}