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/VanillaFollowerOracleTests.cs

@@ -12,6 +12,11 @@ namespace SVSim.BattleEngine.Tests
     [TestFixture]
     public class VanillaFollowerOracleTests
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+        [TearDown] public void TearDownScope() { _scope?.Dispose(); _scope = null; }
+
         private static void SetPrivateField(object obj, string name, object value)
         {
             var f = obj.GetType().GetField(name,
@@ -26,9 +31,9 @@ namespace SVSim.BattleEngine.Tests
         [Test]
         public void Vanilla_follower_resolves_to_correct_state()
         {
-            HeadlessEngineEnv.EnsureInitialized();
             BattleManagerBase.IsForecast = true;           // suppress VFX registration (F1)
             var mgr = new SingleBattleMgr(new HeadlessContentsCreator());
+            _scope.Ctx.Mgr = mgr;
             mgr.IsRecovery = true;                          // collapse wait delays to 0 (F1)
 
             var player = mgr.BattlePlayer;