refactor(engine-ambient): GameMgr.GetIns throws Require; wrap SessionBattleEngine entry points

Step 5 of multi-instancing migration. GameMgr.GetIns() now resolves through
BattleAmbient.Require() (throws when no scope active — fail-fast since engine
callers unconditionally dereference). SessionBattleEngine now owns a single
BattleAmbientContext, pushed via BattleAmbient.Enter at Setup/Receive/all
~30 read accessors and Debug* seams.

EngineGlobalInit.WirePerSessionGameMgr extracted out of the _done-gated block:
GameMgr is now per-session (ctx.GameMgr is a fresh `new()` per SessionBattleEngine),
so the DataMgr chara ids + NetworkUserInfoData seeding must run every Setup, not
process-once. The wiring itself is already idempotent. Without this, second-or-
later sessions in a process NRE in NetworkBattleManagerBase.CreateBackgroundId.

Expected state: SVSim.BattleEngine.Tests have known-failing tests that don't
go through SessionBattleEngine (Task 6 wraps HeadlessFixture). SVSim.UnitTests
mostly recover; residual failures (deal-frame Accepted:false in conductor
integration tests) are captured in
data_dumps/task5-test-output/failing-tests-after-task5-node-postwrap.txt for
Task 7.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SVSim.BattleEngine.Tests/BattleAmbientTests.cs

@@ -200,4 +200,41 @@ public class BattleAmbientTests
         Wizard.Data.BattleRecoveryInfo = info;
         Assert.That(ctx.RecoveryInfo, Is.SameAs(info));
     }
+
+    [Test]
+    public void GameMgr_GetIns_InsideScope_ReturnsScopeInstance()
+    {
+        var mgr = new GameMgr();
+        var ctx = new BattleAmbientContext { GameMgr = mgr };
+        using var _ = BattleAmbient.Enter(ctx);
+        Assert.That(GameMgr.GetIns(), Is.SameAs(mgr));
+    }
+
+    [Test]
+    public void GameMgr_GetIns_OutsideScope_Throws()
+    {
+        Assert.That(BattleAmbient.Current, Is.Null);
+        Assert.Throws<System.InvalidOperationException>(() => GameMgr.GetIns());
+    }
+
+    [Test]
+    public async Task GameMgr_GetIns_IsolatedBetweenConcurrentTasks()
+    {
+        var mgrA = new GameMgr();
+        var mgrB = new GameMgr();
+
+        var taskA = Task.Run(async () => {
+            using var _ = BattleAmbient.Enter(new BattleAmbientContext { GameMgr = mgrA });
+            await Task.Delay(20);
+            return GameMgr.GetIns();
+        });
+        var taskB = Task.Run(async () => {
+            using var _ = BattleAmbient.Enter(new BattleAmbientContext { GameMgr = mgrB });
+            await Task.Delay(20);
+            return GameMgr.GetIns();
+        });
+        var results = await Task.WhenAll(taskA, taskB);
+        Assert.That(results[0], Is.SameAs(mgrA));
+        Assert.That(results[1], Is.SameAs(mgrB));
+    }
 }