Multi-instancing migration (Step 2): convert the two static `bool` fields IsForecast and
IsRandomDraw to static properties that resolve through BattleAmbient.Current when a scope is
active, falling back to a private static when not (preserves today's behavior for unwrapped
callers — solo SingleBattleMgr, in-process unit tests). This is the field-to-property pivot
that lets HeadlessBattleMgr (and any future per-session battle host) carry its own forecast /
random-draw flags inside an AsyncLocal scope without colliding with sibling battles
(design 2026-06-07-engine-multi-instancing, Task 2). ZERO logic change for unwrapped callers
(fallback storage holds the value); scoped callers get per-scope isolation.

--- Engine/BattleManagerBase.cs  (~line 414)
-	public static bool IsRandomDraw = false;
+	private static bool _isRandomDrawFallback = false;
+	public static bool IsRandomDraw {
+		get => SVSim.BattleEngine.Ambient.BattleAmbient.Current?.IsRandomDraw ?? _isRandomDrawFallback;
+		set {
+			var c = SVSim.BattleEngine.Ambient.BattleAmbient.Current;
+			if (c != null) c.IsRandomDraw = value;
+			else _isRandomDrawFallback = value;
+		}
+	}

--- Engine/BattleManagerBase.cs  (~line 416)
-	public static bool IsForecast = false;
+	private static bool _isForecastFallback = false;
+	public static bool IsForecast {
+		get => SVSim.BattleEngine.Ambient.BattleAmbient.Current?.IsForecast ?? _isForecastFallback;
+		set {
+			var c = SVSim.BattleEngine.Ambient.BattleAmbient.Current;
+			if (c != null) c.IsForecast = value;
+			else _isForecastFallback = value;
+		}
+	}