feat(battlenode): opponent reveal resolves on engine state via ReplaceReceivedCards (M-HC-2)

Drive a node-native battle to seat B's turn, then ingest an opponent
PlayActions reveal frame (knownList[{idx,cardId,to:Field}], isPlayerSeat:false)
matching battle_test_cl2's wire shape. The engine's ReplaceReceivedCard.ReplaceCard
-> CreateActualCard -> CreateBattleCardWithGameObject path resolves headless and
seats the substituted card on seat B's board with the wire cardId. No Engine/ logic
edits and no new view shims were needed — the card-creation view surface is fully
covered by the BackGround/icon-anim/play-queue/hand stubs from Tasks 2/3.

Adds InPlayCardId(seat, boardPos) accessor (SessionBattleEngine + harness) to read a
seated in-play follower's true identity, leader-excluded like BoardCount.

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

SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs

@@ -161,6 +161,14 @@ internal sealed class SessionBattleEngine
     /// a card dealt from the seeded deck.</summary>
     public int HandCardIndex(bool playerSeat, int handPos) => Seat(playerSeat).HandCardList[handPos].Index;
 
+    /// <summary>The real <c>CardId</c> (wire identity) of the in-play follower at <paramref name="boardPos"/>
+    /// (0-based, skipping the leader/Class card at ClassAndInPlayCardList[0] — same convention as
+    /// <see cref="BoardCount"/>). Used to assert an opponent reveal seated the substituted card with its
+    /// true identity (M-HC-2): before the reveal the slot holds a hidden dummy (cardId 0); after, the
+    /// engine-resolved actual card carries the wire cardId.</summary>
+    public int InPlayCardId(bool playerSeat, int boardPos) =>
+        Seat(playerSeat).ClassAndInPlayCardList[boardPos + 1].CardId;
+
     private engine::BattlePlayerBase Seat(bool playerSeat) =>
         (_mgr ?? throw new InvalidOperationException("read before Setup")).GetBattlePlayer(playerSeat);