feat(battlenode): Receive ingests a captured PlayActions headless (Phase 2 N0)

Receive feeds the decoded frame into the mgr's own NetworkBattleReceiver
(isHaveSequence:true, checkBreakData:false — mirroring the engine's
RecoveryDataHandler frame replay), reboxing object?->object for nested data.
No engine gaps surfaced; the only fix was a test-harness one (load all deck ids
in a single HeadlessCardMaster.Load — per-id calls each replace the master).

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

SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs

@@ -2,6 +2,8 @@ extern alias engine;
 using System.Reflection;
 using engine::SVSim.BattleEngine.Rng;
 using SVSim.BattleNode.Protocol;
+using NetworkBattleReceiver = engine::NetworkBattleReceiver;
+using NetworkBattleDefine = engine::NetworkBattleDefine;
 using BattleManagerBase = engine::BattleManagerBase;
 using BattlePlayerBase = engine::BattlePlayerBase;
 using BattleCardBase = engine::BattleCardBase;
@@ -28,6 +30,7 @@ internal sealed class SessionBattleEngine
     private const int DefaultLeaderLife = 20;
 
     private HeadlessNetworkBattleMgr? _mgr;
+    private NetworkBattleReceiver? _receiver;
 
     /// <summary>True once Setup has built the two-seat battle.</summary>
     public bool IsReady => _mgr is not null;
@@ -58,12 +61,61 @@ internal sealed class SessionBattleEngine
         SeedDeck(mgr, seatBDeck, isPlayer: false);
 
         _mgr = mgr;
+        // Use the mgr's OWN receiver — the ctor already wired it to the mgr's OperateReceive +
+        // NetworkBattleData (NetworkBattleManagerBase.cs:266, non-recovery branch). This is the same
+        // receiver the engine's RecoveryDataHandler drives when replaying recorded frames.
+        _receiver = mgr.GetNetworkBattleReceiver();
     }
 
     /// <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>
+    /// maps the sender to the engine's player(true)/opponent(false) seat (F-N-2). A throw/reject is
+    /// returned as a detected-desync EVENT (ND6), never silently absorbed.</summary>
     public EngineIngestResult Receive(MsgEnvelope env, bool isPlayerSeat)
-        => throw new NotImplementedException("Filled by Task 4 (ingest probe).");
+    {
+        if (_mgr is null || _receiver is null)
+            throw new InvalidOperationException("Receive before Setup.");
+
+        var dict = ToEngineDict((env.Body as RawBody)?.Entries);
+        var uri = MapUri(env.Uri);
+
+        try
+        {
+            // Mirror the engine's own recorded-frame replay (RecoveryDataHandler.cs:283): every
+            // ingested action resolves through the isHaveSequence ConductReceiveData path, and
+            // checkBreakData:false so a partial/handshake frame is not rejected as a break.
+            bool accepted = _receiver.ReceivedMessage(
+                uri, isHaveSequence: true, dict, isPlayerSeat, handler: null, checkBreakData: false);
+            return accepted ? EngineIngestResult.Ok() : EngineIngestResult.Reject($"receiver rejected {env.Uri}");
+        }
+        catch (Exception ex)
+        {
+            return EngineIngestResult.Reject($"{env.Uri} threw: {ex.GetType().Name}: {ex.Message}");
+        }
+    }
+
+    private static NetworkBattleDefine.NetworkBattleURI MapUri(NetworkBattleUri uri)
+        => Enum.Parse<NetworkBattleDefine.NetworkBattleURI>(uri.ToString());
+
+    // The receiver reads keys via Enum.IsDefined over NetworkParameter and casts nested values to
+    // List<object> / Dictionary<string,object>; the node decodes nested data as the nullable
+    // List<object?> / Dictionary<string,object?>. Rebox to the non-nullable shape, dropping nulls
+    // (the receiver presence-checks keys, so an absent key is the correct encoding of a null).
+    private static Dictionary<string, object> ToEngineDict(Dictionary<string, object?>? entries)
+    {
+        var result = new Dictionary<string, object>();
+        if (entries is null) return result;
+        foreach (var (k, v) in entries)
+            if (v is not null) result[k] = Rebox(v);
+        return result;
+    }
+
+    private static object Rebox(object v) => v switch
+    {
+        Dictionary<string, object?> d => d.Where(kv => kv.Value is not null)
+                                          .ToDictionary(kv => kv.Key, kv => Rebox(kv.Value!)),
+        List<object?> l => l.Where(x => x is not null).Select(x => Rebox(x!)).ToList(),
+        _ => v,
+    };
 
     // --- headless wiring (production analogue of HeadlessFixture) -----------------------------------