feat(engine-ambient): add BattleAmbientContext + AsyncLocal scope

Step 1 of the engine multi-instancing migration. Standalone infrastructure;
no engine static reads/writes through it yet. Scope is reentrant (restores
prior on dispose), AsyncLocal flows across awaits, and isolated between
concurrent Task.Run flows.

See docs/superpowers/specs/2026-06-07-engine-multi-instancing-design.md.

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

SVSim.BattleEngine/Shim/BattleAmbientContext.cs

@@ -0,0 +1,48 @@
+// AUTHORED SHIM (not copied). Per-battle ambient context that backs the
+// AsyncLocal singleton seam for multi-instancing (see docs/superpowers/specs/
+// 2026-06-07-engine-multi-instancing-design.md). The engine's per-battle
+// statics (BattleManagerBase.main/IsForecast/IsRandomDraw, GameMgr.GetIns,
+// Certification.viewer_id, ToolboxGame.RealTimeNetworkAgent,
+// Data.BattleRecoveryInfo) resolve through Current when set; process-shared
+// reference data (CardMaster.Default, Data.Master, etc.) stays static.
+#nullable enable
+using System;
+using System.Threading;
+
+namespace SVSim.BattleEngine.Ambient;
+
+public sealed class BattleAmbientContext
+{
+    public BattleManagerBase? Mgr { get; set; }
+    public GameMgr GameMgr { get; init; } = new();
+    public RealTimeNetworkAgent? NetworkAgent { get; set; }
+    public int ViewerId { get; init; } = 1001;
+    public bool IsForecast { get; set; } = true;
+    public bool IsRandomDraw { get; set; } = true;
+    public Wizard.BattleRecoveryInfo? RecoveryInfo { get; set; }
+}
+
+public static class BattleAmbient
+{
+    internal static readonly AsyncLocal<BattleAmbientContext?> _current = new();
+
+    public static BattleAmbientContext? Current => _current.Value;
+
+    public static BattleAmbientContext Require() =>
+        _current.Value ?? throw new InvalidOperationException(
+            "No ambient battle context. Wrap engine entry points in BattleAmbient.Enter(ctx).");
+
+    public static Scope Enter(BattleAmbientContext ctx)
+    {
+        var prior = _current.Value;
+        _current.Value = ctx;
+        return new Scope(prior);
+    }
+
+    public readonly struct Scope : IDisposable
+    {
+        private readonly BattleAmbientContext? _prior;
+        internal Scope(BattleAmbientContext? prior) { _prior = prior; }
+        public void Dispose() => _current.Value = _prior;
+    }
+}