refactor(engine-ambient): IsForecast/IsRandomDraw read ambient first, static fallback

Step 2 of multi-instancing migration. Both flags now resolve through
BattleAmbient.Current when a scope is active, otherwise hit a static fallback
that preserves today's behavior unchanged for unwrapped callers.

Suite green: SVSim.BattleEngine.Tests pass; SVSim.UnitTests baseline holds.

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

SVSim.BattleEngine.Tests/BattleAmbientTests.cs

@@ -81,4 +81,52 @@ public class BattleAmbientTests
         Assert.That(results[0], Is.EqualTo(100));
         Assert.That(results[1], Is.EqualTo(200));
     }
+
+    [Test]
+    public void IsForecast_ReadsAmbient_WhenScopeActive()
+    {
+        var ctx = new BattleAmbientContext { IsForecast = false };
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(BattleManagerBase.IsForecast, Is.False);
+        ctx.IsForecast = true;
+        Assert.That(BattleManagerBase.IsForecast, Is.True);
+    }
+
+    [Test]
+    public void IsForecast_WriteInsideScope_WritesAmbient_NotFallback()
+    {
+        var ctx = new BattleAmbientContext { IsForecast = false };
+        using (var _ = BattleAmbient.Enter(ctx))
+        {
+            BattleManagerBase.IsForecast = true;
+            Assert.That(ctx.IsForecast, Is.True);
+        }
+    }
+
+    [Test]
+    public void IsForecast_OutsideScope_FallsBackToStatic()
+    {
+        Assert.That(BattleAmbient.Current, Is.Null);
+        BattleManagerBase.IsForecast = true;
+        Assert.That(BattleManagerBase.IsForecast, Is.True);
+        BattleManagerBase.IsForecast = false;
+        Assert.That(BattleManagerBase.IsForecast, Is.False);
+    }
+
+    [Test]
+    public void IsRandomDraw_RoundtripsAmbient_And_Fallback()
+    {
+        Assert.That(BattleAmbient.Current, Is.Null);
+        BattleManagerBase.IsRandomDraw = true;
+        Assert.That(BattleManagerBase.IsRandomDraw, Is.True);
+
+        var ctx = new BattleAmbientContext { IsRandomDraw = false };
+        using (var _ = BattleAmbient.Enter(ctx))
+        {
+            Assert.That(BattleManagerBase.IsRandomDraw, Is.False);
+        }
+        Assert.That(BattleManagerBase.IsRandomDraw, Is.True);
+
+        BattleManagerBase.IsRandomDraw = false;
+    }
 }