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/NetworkMgrConstructionProbeTests.cs

@@ -11,13 +11,18 @@ namespace SVSim.BattleEngine.Tests
     [TestFixture]
     public class NetworkMgrConstructionProbeTests : NetworkEmitFixtureBase
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+        [TearDown] public void TearDownScope() { _scope?.Dispose(); _scope = null; }
+
         [Test]
         public void HeadlessNetworkBattleMgr_constructs_headless()
         {
-            HeadlessEngineEnv.EnsureInitialized();
             Assert.DoesNotThrow(() =>
             {
                 var mgr = new HeadlessNetworkBattleMgr(new HeadlessContentsCreator());
+                _scope.Ctx.Mgr = mgr;
                 Assert.That(mgr, Is.Not.Null);
             });
         }
@@ -26,6 +31,7 @@ namespace SVSim.BattleEngine.Tests
         public void OnEmit_capture_seam_is_wired_via_injected_agent()
         {
             var (mgr, emitted) = HeadlessEngineEnv.NewNetworkEmitBattle();
+            _scope.Ctx.Mgr = mgr;
             Assert.That(mgr, Is.Not.Null);
             Assert.That(Wizard.ToolboxGame.RealTimeNetworkAgent, Is.Not.Null,
                 "agent must be injected so NetworkBattleSender's ToolboxGame.RealTimeNetworkAgent.* calls resolve");