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/RandomDrawOracleTests.cs

@@ -16,21 +16,29 @@ namespace SVSim.BattleEngine.Tests
     [TestFixture]
     public class RandomDrawOracleTests
     {
+        private TestBattleScope _scope;
+
+        [SetUp] public void SetUpScope() { _scope = new TestBattleScope(); }
+
         [TearDown]
         public void ResetRandomDrawGate()
         {
             // NewAuthoritativeBattle sets the process-global BattleManagerBase.IsRandomDraw = true; reset it
             // so this fixture doesn't leak that state into later-running fixtures (which expect the default
             // false / top-of-deck draw behavior). Prevents order-dependent flakes as more RNG oracles land.
+            // (Now an ambient write inside the scope; harmless either way.)
             BattleManagerBase.IsRandomDraw = false;
+            _scope?.Dispose();
+            _scope = null;
         }
 
         // Draw with a single scripted unit; return (drawnCardId, deckCountAfter). The deck is seeded with
         // three distinguishable cards at indices 2,3,4 -> Index-order positions 0,1,2 map to
         // RngDeckCardA/B/C. The draw makes one StableRandom(3) call -> index = floor(3*unit).
-        private static (int drawnId, int deckAfter) DrawWith(double unit)
+        private (int drawnId, int deckAfter) DrawWith(double unit)
         {
             var mgr = HeadlessEngineEnv.NewAuthoritativeBattle(new ScriptedRandomSource(new[] { unit }));
+            _scope.Ctx.Mgr = mgr;
             var player = mgr.BattlePlayer;
 
             HeadlessEngineEnv.SeedDeck(mgr, HeadlessEngineEnv.RngDeckCardA, index: 2, isPlayer: true);