test(engine-ambient): TestBattleScope + HeadlessFixture split for multi-instance

Step 6 of multi-instancing migration. HeadlessEngineEnv.EnsureInitialized
is split into EnsureProcessGlobals (idempotent, process-once) +
SeedCharaIdsOnCurrentAmbient (per-test). New TestBattleScope IDisposable
sets up a fresh BattleAmbientContext per test. NonParallelizable removed
from converted classes; assembly-level Parallelizable(Fixtures) enabled.

SVSim.BattleEngine.Tests fully green under parallel test execution.

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

SVSim.BattleEngine.Tests/ConstructionProbeTests.cs

@@ -14,12 +14,17 @@ namespace SVSim.BattleEngine.Tests
     [TestFixture]
     public class ConstructionProbeTests
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+        [TearDown] public void TearDownScope() { _scope?.Dispose(); _scope = null; }
+
         [Test]
         public void SingleBattleMgr_constructs_headless()
         {
             // Mirror the forecast flags the design pins (DP4 / §3): suppress VFX registration and
-            // collapse wait delays. Set before construction so any ctor-time VFX path no-ops.
-            HeadlessEngineEnv.EnsureInitialized();
+            // collapse wait delays. TestBattleScope already sets ctx.IsForecast=true; this line is a
+            // belt-and-suspenders write through the ambient setter.
             BattleManagerBase.IsForecast = true;
 
             SingleBattleMgr mgr = null;
@@ -37,6 +42,7 @@ namespace SVSim.BattleEngine.Tests
             Assert.That(mgr, Is.Not.Null);
             Assert.That(mgr.BattlePlayer, Is.Not.Null, "BattlePlayer (self) not created");
             Assert.That(mgr.BattleEnemy, Is.Not.Null, "BattleEnemy (opponent) not created");
+            _scope.Ctx.Mgr = mgr;
         }
     }
 }