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/SessionEngine/SessionEngineConstructionTests.cs

@@ -7,6 +7,11 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
     [TestFixture]
     public class SessionEngineConstructionTests
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+        [TearDown] public void TearDownScope() { _scope?.Dispose(); _scope = null; }
+
         [Test]
         public void SessionBattleEngine_instantiates_and_is_not_ready_before_setup()
         {
@@ -17,7 +22,6 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
         [Test]
         public void Setup_builds_two_seat_network_battle_headless()
         {
-            HeadlessEngineEnv.EnsureInitialized();
             // Load every card id the two test decks reference so CardMaster can resolve them.
             var deckA = Enumerable.Repeat(100011010L, 40).ToList(); // vanilla 1/2 follower x40
             var deckB = Enumerable.Repeat(100011010L, 40).ToList();
@@ -48,8 +52,6 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
                 "draw-misalignment makes a captured play unresolvable against a node-seated deck; the " +
                 "node-native harness is the post-M-HC-0b oracle. Revive if capture-replay alignment lands.");
 
-            HeadlessEngineEnv.EnsureInitialized();
-
             var cl1 = CaptureReplay.Load("battle_test_cl1.ndjson");
             var deck = CaptureReplay.SelfDeckFrom(cl1);
             // Load ALL deck ids in ONE call: HeadlessCardMaster.Load replaces the static CardMaster each