cull(engine-cleanup): pass-8 phase-2 cascade round 2

[trim] fully-unreachable files deleted: 3
[trim] files edited: 21, files deleted: 0, nodes removed: 38
[type-trim] files edited: 3, types removed: 3

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.BattleEngine/Engine/InputMgr.cs

@@ -52,8 +52,6 @@ public class InputMgr
 
 	private IList<GameObject> m_DragEfcList = new List<GameObject>();
 
-	private bool _isBackKeyEnable = true;
-
 	public BattleCamera m_BattleCamera;
 
 	private bool _wentOverDrag;