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/SessionEngineShadowReplayTests.cs

@@ -9,6 +9,11 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
     [TestFixture]
     public class SessionEngineShadowReplayTests
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+        [TearDown] public void TearDownScope() { _scope?.Dispose(); _scope = null; }
+
         // Frames that are transport/keepalive, not game actions — not ingested.
         private static readonly HashSet<string> SkipUris = new()
         {
@@ -35,8 +40,6 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
                 "against a node-seated deck hits the documented draw-misalignment artifact once the " +
                 "receive path actually resolves. Revive if a capture-replay alignment path lands.");
 
-            HeadlessEngineEnv.EnsureInitialized();
-
             var cl1 = CaptureReplay.Load("battle_test_cl1.ndjson");
             var cl2 = CaptureReplay.Load("battle_test_cl2.ndjson");
             var deckA = CaptureReplay.SelfDeckFrom(cl1);