refactor(engine-ambient): wrap residual UnitTests + delete EngineSessionGate

Step 7 of multi-instancing migration. Residual SVSim.UnitTests that touch
engine code directly are wrapped in TestBattleScope. EngineSessionGate is
deleted along with the _engineOwned bookkeeping in BattleSession; engine
setup is unconditional now that per-battle state is isolated on the ambient.
Gate-specific fallback branches in BattleSession.ShadowIngest are simplified.

Suite fully green (SVSim.UnitTests, SVSim.BattleEngine.Tests).

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

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -52,11 +52,6 @@ public sealed class BattleSession
     /// was already processed during the Swap feed.</summary>
     private bool _engineReadyFed;
 
-    /// <summary>True once this session has acquired the process-wide <see cref="Engine.EngineSessionGate"/>
-    /// (and is therefore the single active engine owner). Drives the matching <c>Release</c> at battle
-    /// end so the next session can take the engine.</summary>
-    private bool _engineOwned;
-
     /// <summary>Serializes dispatch. Both participants' read loops raise FrameEmitted on their own
     /// threads, and a dispatch (<see cref="ComputeFrames"/> + the relay <c>PushAsync</c> calls) mutates
     /// shared, non-thread-safe state — the <see cref="BattleSessionState"/> dictionaries and each
@@ -191,24 +186,18 @@ public sealed class BattleSession
         if (A is RealParticipant rpA) rpA.Outbound.Clear();
         if (B is RealParticipant rpB) rpB.Outbound.Clear();
 
-        try
-        {
-            await Task.WhenAll(
-                A.TerminateAsync(BattleFinishReason.NormalFinish),
-                B.TerminateAsync(BattleFinishReason.NormalFinish))
-                .ConfigureAwait(false);
+        // Per-session BattleAmbientContext on the engine isolates per-battle state across concurrent
+        // sessions (Task 7 of multi-instancing migration), so the historical single-active-engine gate
+        // (and its matching try/finally Release) is gone — engine setup is unconditional per session,
+        // and there is no teardown obligation that must run on a throw from the participant tear-down.
+        await Task.WhenAll(
+            A.TerminateAsync(BattleFinishReason.NormalFinish),
+            B.TerminateAsync(BattleFinishReason.NormalFinish))
+            .ConfigureAwait(false);
 
-            await A.DisposeAsync().ConfigureAwait(false);
-            await B.DisposeAsync().ConfigureAwait(false);
-            _dispatchGate.Dispose();
-        }
-        finally
-        {
-            // Release the single-active-engine gate exactly once, in a finally so a throw from the
-            // terminate/dispose teardown above can never leak it to the next session (the gate is a
-            // process-global static shared across all sessions, incl. across tests in one process).
-            if (_engineOwned) Engine.EngineSessionGate.Release();
-        }
+        await A.DisposeAsync().ConfigureAwait(false);
+        await B.DisposeAsync().ConfigureAwait(false);
+        _dispatchGate.Dispose();
     }
 
     private Task OnFrameFromA(MsgEnvelope env, CancellationToken ct) => HandleFrameAsync(A, env, ct);
@@ -349,18 +338,13 @@ public sealed class BattleSession
         if (_engineSetupAttempted) return;
         _engineSetupAttempted = true;
 
-        // Single-active-engine gate: the engine's process-global turn state can't back two concurrent
-        // battles, so only one session may own it (carried-risk B). On failure we DON'T set the engine
-        // up — it stays not-ready and ShadowIngest no-ops on !IsReady — and log the limitation loudly
-        // (not a silent fallback). Per-session isolation (dropping the gate) is the tracked follow-up.
-        if (!Engine.EngineSessionGate.TryAcquire())
-        {
-            _log.LogWarning("BattleSession {Bid}: another battle owns the engine; this battle runs " +
-                "WITHOUT engine-sourced fields (single-active-engine limitation — per-session isolation pending)",
-                BattleId);
-            return;
-        }
-        _engineOwned = true;
+        // Per-session BattleAmbientContext on the engine isolates per-battle state across concurrent
+        // sessions (Task 7 of multi-instancing migration), so the historical single-active-engine gate
+        // (EngineSessionGate.TryAcquire) is gone and engine setup is unconditional. A genuine setup
+        // failure still surfaces via ComputeFrames' shadow-engine try/catch (it logs + swallows so the
+        // relay never sees an engine exception, ND6), and IsReady stays false in that case so
+        // ShadowIngest/ShadowFeedServerFrames no-op for the rest of the battle.
+        //
         // Seed the engine's StableRandom with BattleSeeds.Stable(MasterSeed) — the SAME value the
         // Matched frame ships to both clients (InitBattleHandler.cs:28). The clients seed their
         // System.Random with Matched.seed (BattleManagerBase.cs:721), so the engine's stream must

SVSim.BattleNode/Sessions/Dispatch/Handlers/PlayActionsHandler.cs

@@ -50,8 +50,8 @@ internal sealed class PlayActionsHandler : IFrameHandler
         // engine.Receive for THIS frame before this handler runs, so the engine has resolved the play and
         // PlayedCardCost reads the discounted cost it actually charged (spellboost + board modifiers folded
         // in BY CONSTRUCTION — no bookkeeping). Sender's seat == ctx.A (BattleSession.ShadowIngest uses the
-        // same ReferenceEquals(from, A) mapping). Degrades to 0 when the engine isn't owned/ready for this
-        // session (single-active-engine gate) so a non-engine session never crashes.
+        // same ReferenceEquals(from, A) mapping). Degrades to 0 when the engine isn't ready for this session
+        // (Setup failed and the ComputeFrames try/catch swallowed it, ND6) so a non-engine session never crashes.
         bool senderSeat = ReferenceEquals(ctx.From, ctx.A);
         int playedCost = ctx.Engine.PlayedCardCost(senderSeat, playIdx, fallback: 0);
 

SVSim.BattleNode/Sessions/Engine/EngineSessionGate.cs

@@ -1,17 +0,0 @@
-namespace SVSim.BattleNode.Sessions.Engine;
-
-/// <summary>Process-wide single-active-engine gate. The engine's process-global turn state
-/// (ToolboxGame.RealTimeNetworkAgent, GameMgr) cannot safely back two concurrent battles, so exactly
-/// one BattleSession may own the engine at a time (AskUserQuestion 2026-06-06: serialize + document).
-/// A session that cannot acquire runs WITHOUT the engine and logs it loudly — NOT a silent fallback
-/// (the operator sees the limitation). Per-session isolation (removing this gate) is the tracked
-/// follow-up. Non-blocking TryAcquire keeps it out of the synchronous dispatch path; in local
-/// single-user dev battles are sequential, so contention never arises.</summary>
-internal static class EngineSessionGate
-{
-    private static int _owned; // 0 = free, 1 = owned
-
-    public static bool TryAcquire() => System.Threading.Interlocked.CompareExchange(ref _owned, 1, 0) == 0;
-
-    public static void Release() => System.Threading.Interlocked.Exchange(ref _owned, 0);
-}

SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs

@@ -53,6 +53,15 @@ internal sealed class SessionBattleEngine
         ViewerId = EngineGlobalInit.ThisViewerId,
         IsForecast = true,
         IsRandomDraw = true,
+        // Per-session BattleRecoveryInfo: the receive-conductor deal path runs under IsRecovery
+        // (set after mgr construction below) and reads Data.BattleRecoveryInfo.IsMulliganEnd in
+        // MulliganMgrBase.StartDeal — null reads NRE. Each session owns its own no-op instance with
+        // IsMulliganEnd=false (the default); GetUninitializedObject skips the JsonData ctor. Each
+        // SessionBattleEngine carries its own ambient _ctx, so per-session isolation is by construction
+        // (the EngineGlobalInit fallback only seeded once-per-process and silently fell over for the
+        // second + later session that entered a fresh ambient — diagnosed Task 7).
+        RecoveryInfo = (engine::Wizard.BattleRecoveryInfo)FormatterServices
+            .GetUninitializedObject(typeof(engine::Wizard.BattleRecoveryInfo)),
     };
 
     private HeadlessNetworkBattleMgr? _mgr;
@@ -68,9 +77,11 @@ internal sealed class SessionBattleEngine
     /// the <c>CardClass</c> int value); they select the leader's class via the all-8-class
     /// ClassCharacterList EngineGlobalInit installs (chara_id == class_id for 1..8). The 3-arg overload
     /// behavior is preserved by the defaults (1/2), matching the test-harness charaIds.
-    /// <para>NOTE: GameMgr (the leader chara ids set below) is a PROCESS GLOBAL. Setting per-session
-    /// chara ids is therefore only safe while exactly one engine-backed battle exists at a time — the
-    /// invariant <see cref="EngineSessionGate"/> enforces on the caller side.</para></summary>
+    /// <para>NOTE: GameMgr is now per-session via <see cref="BattleAmbientContext.GameMgr"/>; the leader
+    /// chara ids are set on the SESSION's GameMgr (resolved through the ambient by
+    /// <c>EngineGlobalInit.WirePerSessionGameMgr</c>), not on a process-wide singleton. This is the Task-7
+    /// payoff: concurrent sessions each own their own GameMgr + engine state, so the historical
+    /// single-active-engine gate (deleted EngineSessionGate) is no longer needed.</para></summary>
     public void Setup(int masterSeed,
         IReadOnlyList<long> seatADeck, IReadOnlyList<long> seatBDeck,
         int seatAClass = 1, int seatBClass = 2)
@@ -155,8 +166,8 @@ internal sealed class SessionBattleEngine
         InstallHeadlessNetworkAgent(); // turn-flow resolve reads ToolboxGame.RealTimeNetworkAgent
 
         // Per-session leader class: chara_id == class_id for 1..8 in the all-8-class ClassCharacterList,
-        // so writing the seats' class ordinals into GameMgr's DataMgr resolves each leader's correct
-        // class. Process-global — safe only under EngineSessionGate (see method remarks above).
+        // so writing the seats' class ordinals into the SESSION's GameMgr DataMgr (resolved through the
+        // ambient — see Setup remarks) resolves each leader's correct class.
         SetGameMgrCharaIds(seatAClass, seatBClass);
 
         SeedDeck(mgr, seatADeck, isPlayer: true);
@@ -340,12 +351,12 @@ internal sealed class SessionBattleEngine
     //
     // INVARIANT (two accessor bands, different null-engine policy):
     //   • This "oracle" band (down to EvolveWaitTurnCount) goes through Seat(), which THROWS if the
-    //     engine isn't owned/seated for this session. It is TEST-ONLY — called solely from the
+    //     engine isn't seated for this session. It is TEST-ONLY — called solely from the
     //     node-native harness/tests, where the engine is always seated. Do NOT call these from a wire
     //     handler.
     //   • The wire-path band below (PlayedCardCost/Spellboost/Clan/Tribe/Id) DEGRADES to a fallback
-    //     when the engine isn't owned (single-active-engine gate), so a non-engine session never
-    //     crashes. Production handlers read ONLY that band.
+    //     when _mgr is null (Setup failed and the ComputeFrames try/catch swallowed it, ND6), so a
+    //     non-engine session never crashes. Production handlers read ONLY that band.
 
     public int LeaderLife(bool playerSeat) { using var _ambient = BattleAmbient.Enter(_ctx); return Seat(playerSeat).Class.Life; }
     public int Pp(bool playerSeat) { using var _ambient = BattleAmbient.Enter(_ctx); return Seat(playerSeat).Pp; }
@@ -449,8 +460,8 @@ internal sealed class SessionBattleEngine
     /// so <see cref="BattleCardBase.PlayedCost"/> is the authoritative play-time discounted cost. We search
     /// the seat's post-resolution zones (in-play, cemetery) by <c>Index</c>, then fall back to the hand
     /// (a not-yet-resolved card, e.g. a degenerate test path) reading the live <c>Cost</c> there.</para>
-    /// <para>Degrades to <paramref name="fallback"/> when the engine is not set up (the single-active-engine
-    /// gate left this session without an owned engine) or the idx resolves to no card — so a non-engine
+    /// <para>Degrades to <paramref name="fallback"/> when the engine is not set up (Setup failed and the
+    /// ComputeFrames try/catch swallowed it, ND6) or the idx resolves to no card — so a non-engine
     /// session never crashes and a vanilla play simply emits its base cost via the caller's fallback.</para></summary>
     public int PlayedCardCost(bool playerSeat, int idx, int fallback = 0)
     {
@@ -500,7 +511,7 @@ internal sealed class SessionBattleEngine
     /// </list>
     /// Same post-resolution zone search + degrade-to-<paramref name="fallback"/> contract as
     /// <see cref="PlayedCardCost"/>: no engine / no card → <paramref name="fallback"/>, so a non-engine session
-    /// (the single-active-engine gate left this session without an owned engine) keeps emitting the deck-map id via
+    /// (Setup failed and the ComputeFrames try/catch swallowed it, ND6) keeps emitting the deck-map id via
     /// the caller's fallback, never crashing.</summary>
     public long PlayedCardId(bool playerSeat, int idx, long fallback = 0)
     {
@@ -538,10 +549,10 @@ internal sealed class SessionBattleEngine
     /// <see cref="PlayedCardClan"/>: no engine / no card → <paramref name="fallback"/> (default <c>"0"</c>, the
     /// prod no-tribe form — NEVER empty, which is wire-illegal: prod always sends tribe as a non-empty string,
     /// the client reads it via <c>item.Value.ToString()</c> at NetworkBattleReceiver.cs:2382). The degrade is
-    /// LIVE, not dead: a second concurrent battle that loses the single-active-engine gate has <c>_mgr is null</c>
-    /// yet still emits a knownList entry (the handler resolves the identity via the deck-map/mined fallback when
-    /// the engine read degrades, so BuildPlayedCard still synthesizes an entry), so this path must hand back a
-    /// legal wire value.</para></summary>
+    /// LIVE, not dead: a session whose Setup failed (the ComputeFrames try/catch swallowed it, ND6) has
+    /// <c>_mgr is null</c> yet still emits a knownList entry (the handler resolves the identity via the
+    /// deck-map/mined fallback when the engine read degrades, so BuildPlayedCard still synthesizes an
+    /// entry), so this path must hand back a legal wire value.</para></summary>
     public string PlayedCardTribe(bool playerSeat, int idx, string fallback = "0")
     {
         using var _ambient = BattleAmbient.Enter(_ctx);
@@ -923,12 +934,11 @@ internal sealed class SessionBattleEngine
         ToolboxGame.SetRealTimeNetworkBattle(agent);
     }
 
-    // Write the two seats' class ordinals into GameMgr's DataMgr leader chara ids. Mirrors the test
-    // seam HeadlessFixture.cs:202-204 (SetField(dm, "_playerCharaId"/"_enemyCharaId", ...)). chara_id ==
-    // class_id for 1..8 in EngineGlobalInit's all-8-class ClassCharacterList, so the ordinal selects the
-    // class. A non-positive ordinal (e.g. CardClass.None == 0) clamps to the default seat (1/2).
-    // GameMgr is a process global → safe only under EngineSessionGate (one engine-backed battle at a
-    // time).
+    // Write the two seats' class ordinals into the SESSION's GameMgr DataMgr leader chara ids. Mirrors
+    // the test seam HeadlessFixture.cs:202-204 (SetField(dm, "_playerCharaId"/"_enemyCharaId", ...)).
+    // chara_id == class_id for 1..8 in EngineGlobalInit's all-8-class ClassCharacterList, so the ordinal
+    // selects the class. A non-positive ordinal (e.g. CardClass.None == 0) clamps to the default seat
+    // (1/2). GameMgr is per-session (BattleAmbientContext.GameMgr); writes resolve through the ambient.
     private static void SetGameMgrCharaIds(int a, int b)
     {
         var dm = GameMgr.GetIns().GetDataMgr();