feat(battlenode): SessionBattleEngine skeleton + types (Phase 2 N0)

SessionContentsCreator mirrors the test HeadlessContentsCreator fully (all
IBattleMgrContentsCreator members) so it compiles; Setup/Receive throw pending
the Task 3/4 probes. New files use the 'engine' extern alias.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/Engine/EngineIngestResult.cs

@@ -0,0 +1,9 @@
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// <summary>Outcome of feeding one client frame to the engine (design ND6). A divergence/reject is a
+/// DETECTED-DESYNC EVENT surfaced to the caller — never silently absorbed. Phase-2 policy: log.</summary>
+internal sealed record EngineIngestResult(bool Accepted, bool Diverged, string? RejectReason)
+{
+    public static EngineIngestResult Ok() => new(Accepted: true, Diverged: false, RejectReason: null);
+    public static EngineIngestResult Reject(string reason) => new(Accepted: false, Diverged: true, RejectReason: reason);
+}

SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs

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+extern alias engine;
+using engine::SVSim.BattleEngine.Rng;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// <summary>One authoritative engine per BattleSession, seated as both players (design ND2). A faithful
+/// SHADOW: it mirrors each client's resolved play, never overrides/rejects/originates (ND1). Ingest is
+/// the engine's own NetworkBattleReceiver.ReceivedMessage (ND4); isPlayer selects the seat (F-N-2).</summary>
+internal sealed class SessionBattleEngine
+{
+    private HeadlessNetworkBattleMgr? _mgr;
+
+    /// <summary>True once Setup has built the two-seat battle.</summary>
+    public bool IsReady => _mgr is not null;
+
+    /// <summary>Construct the two-seat network battle from both decks + the master seed (design F-N-5).
+    /// <paramref name="seatADeck"/>/<paramref name="seatBDeck"/> are the per-side deck orders the node
+    /// already computed (BattleSessionState.GetShuffledDeck) and handed each client.</summary>
+    public void Setup(int masterSeed,
+        IReadOnlyList<long> seatADeck, IReadOnlyList<long> seatBDeck)
+        => throw new NotImplementedException("Filled by Task 3 (construction probe).");
+
+    /// <summary>Ingest one client frame into the engine for the given seat. <paramref name="isPlayerSeat"/>
+    /// maps the sender to the engine's player(true)/opponent(false) seat (F-N-2).</summary>
+    public EngineIngestResult Receive(MsgEnvelope env, bool isPlayerSeat)
+        => throw new NotImplementedException("Filled by Task 4 (ingest probe).");
+}

SVSim.BattleNode/Sessions/Engine/SessionContentsCreator.cs

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+extern alias engine;
+using engine::Wizard.BattleMgr;
+using engine::Wizard.Battle.Phase;
+using engine::Wizard.Battle.Recovery;
+using engine::Wizard.Battle.Replay;
+using engine::Wizard.Battle.Resource;
+using engine::Wizard.Battle.View.Vfx;
+
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// <summary>The node's production IBattleMgrContentsCreator. Mirrors the test-side
+/// HeadlessContentsCreator (HeadlessFixture.cs) but carries the per-battle master seed so the
+/// engine's RNG stream is born aligned with the seed the node handed both clients (design F-N-5).
+/// The non-RandomSeed members are the no-op recovery/replay/resource/vfx/phase creators the
+/// NetworkStandardBattleMgr ctor dereferences — the engine's own Null* implementations, same set the
+/// headless test harness uses.</summary>
+internal sealed class SessionContentsCreator : IBattleMgrContentsCreator
+{
+    public SessionContentsCreator(int masterSeed) => RandomSeed = masterSeed;
+
+    public int RandomSeed { get; }
+
+    // No-op managers: the ctor's FirstRecoverySetting/FirstReplaySetting dereference these; recovery/
+    // replay recording is irrelevant to a shadow engine, so use the engine's own null implementations.
+    public IRecoveryManager RecoveryManager { get; } = new NullRecoveryManager();
+    public IRecoveryRecordManager RecoveryRecordManager { get; } = new NullRecoveryRecordManager();
+    public IReplayRecordManager ReplayRecordManager { get; } = new NullReplayRecordManager();
+
+    public IBattleResourceMgr CreateResourceMgr() => new BattleResourceMgr();
+    public VfxMgr CreateVfxMgr() => new VfxMgr();
+    public IPhaseCreator CreatePhaseCreator(engine::BattleManagerBase battleMgr) =>
+        new SessionPhaseCreator(battleMgr);
+}
+
+/// <summary>Node analogue of the test HeadlessPhaseCreator / the engine's SingleBattlePhaseCreator
+/// (cut from the M1 copy set as an entry-point ctor): inherits PhaseCreatorBase wholesale.</summary>
+internal sealed class SessionPhaseCreator : PhaseCreatorBase
+{
+    public SessionPhaseCreator(engine::BattleManagerBase battleMgr) : base(battleMgr) { }
+}