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/TestBattleScope.cs

@@ -0,0 +1,50 @@
+#nullable enable
+using System;
+using System.Runtime.Serialization;
+using SVSim.BattleEngine.Ambient;
+
+namespace SVSim.BattleEngine.Tests;
+
+/// <summary>Per-test ambient scope. Each test that touches engine statics wraps its body
+/// in `using var scope = new TestBattleScope();` (or with an explicit Mgr/ViewerId).
+///
+/// The constructor enters a fresh <see cref="BattleAmbientContext"/> (carrying a brand-new
+/// <see cref="GameMgr"/> so per-test mgr/DataMgr writes never bleed across tests), then
+/// runs the per-ambient seeders that <see cref="HeadlessEngineEnv.EnsureProcessGlobals"/>
+/// no longer does (chara ids on DataMgr, NetworkUserInfoData). Process-globals
+/// (card master, LoadDetail, Crossover, Certification.udid) come from
+/// <see cref="HeadlessEngineEnv.EnsureProcessGlobals"/> which runs once per process.
+///
+/// Public surface (vs. internal) so SVSim.UnitTests can reuse it via the same project
+/// reference in Task 7.</summary>
+public sealed class TestBattleScope : IDisposable
+{
+    private readonly BattleAmbient.Scope _scope;
+    public BattleAmbientContext Ctx { get; }
+
+    public TestBattleScope(BattleManagerBase? mgr = null, int viewerId = 1001)
+    {
+        // Make sure process-globals are seeded before we enter; idempotent + cheap after first call.
+        HeadlessEngineEnv.EnsureProcessGlobals();
+
+        Ctx = new BattleAmbientContext
+        {
+            Mgr = mgr,
+            GameMgr = new GameMgr(),
+            ViewerId = viewerId,
+            IsForecast = true,
+            IsRandomDraw = true,
+            RecoveryInfo = (Wizard.BattleRecoveryInfo)FormatterServices
+                .GetUninitializedObject(typeof(Wizard.BattleRecoveryInfo)),
+        };
+        _scope = BattleAmbient.Enter(Ctx);
+
+        // Per-ambient seeders MUST run AFTER scope entry so GameMgr.GetIns() resolves to this
+        // scope's GameMgr (not a stray one). EnsureProcessGlobals used to do these writes against
+        // the global GameMgr; now they're scoped.
+        HeadlessEngineEnv.SeedCharaIdsOnCurrentAmbient();
+        HeadlessEngineEnv.SeedNetUserOnCurrentAmbient();
+    }
+
+    public void Dispose() => _scope.Dispose();
+}