feat(battlenode): emit engine-resolved cost on every knownList entry (M-HC-3)

The opponent-facing PlayActions knownList now carries the engine-RESOLVED
play-time cost (KnownCardEntry.cost), sourced from the headless shadow engine's
PlayedCost on the just-resolved card. This closes the spellboost cost-desync BY
CONSTRUCTION: the engine already knows the true discounted cost (spellboost +
board modifiers folded in), so no bookkeeping is needed.

- DTO: add non-nullable cost to KnownCardEntry (prod emits cost 45/45).
- SessionBattleEngine.PlayedCardCost(seat, idx, fallback): finds the resolved
  card by engine Index across in-play/cemetery/hand zones and returns PlayedCost
  (captured by PlayCard at resolution == discounted Cost), degrading to fallback
  when the engine is not owned/ready.
- PlayActionsHandler sources the played card's cost from ctx.Engine (ShadowIngest
  already resolved the play before the handler runs). Spellboost-map plumbing
  stays for now; Task 6 (M-HC-3b) retires it.
- Validation: engine-read test (charge-seeded reducer 101314020: base 5, cost
  5/1/0 at charge 0/4/5) + handler-emit test asserting knownList[0].cost == 1
  (discounted, not base 5) with non-vacuity. Board-dependent (when_evolve_other)
  case deferred to M-HC-4 (evolve not yet headless); cost is read off the resolved
  engine so board modifiers are captured by construction once their ops resolve.
- Harness: promote alt vanilla follower id (101211120) to AltVanillaFollowerId.

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

SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs

@@ -3,6 +3,7 @@ using SVSim.BattleNode.Protocol;
 using SVSim.BattleNode.Sessions;
 using SVSim.BattleNode.Sessions.Dispatch;
 using SVSim.BattleNode.Sessions.Engine;
+using SVSim.BattleNode.Sessions.Participants;
 
 namespace SVSim.UnitTests.BattleNode.Integration;
 
@@ -55,6 +56,13 @@ internal sealed class NodeNativeBattleHarness : IDisposable
     /// it will produce a traceable failure here.</summary>
     public const long VanillaFollowerId = 100011010;
 
+    /// <summary>A SECOND, distinct cost-1 vanilla follower (char_type 1, cost 1, no skill) — present +
+    /// creatable in cards.json. Used by the opponent-reveal substitution test as the WIRE cardId that
+    /// must override a seeded identity (it is deliberately NOT in any harness deck, so its only route
+    /// onto the board is a reveal). Named here so card-id provenance stays traceable as ids accumulate
+    /// (Task-4 review nit promoted in M-HC-3).</summary>
+    public const long AltVanillaFollowerId = 101211120;
+
     public BattleSessionState State { get; }
     public StubParticipant SeatA { get; }
     public StubParticipant SeatB { get; }
@@ -156,11 +164,18 @@ internal sealed class NodeNativeBattleHarness : IDisposable
     /// (<c>PushAsync</c>, <c>RunAsync</c>, <c>TerminateAsync</c>) throw <see cref="NotSupportedException"/>
     /// — the harness drives the engine directly, so a frame must never reach the participant relay.
     /// Silent no-ops would let a misrouted push pass undetected.</summary>
-    internal sealed class StubParticipant : IBattleParticipant
+    internal sealed class StubParticipant : IBattleParticipant, IHasHandshakePhase
     {
         public long ViewerId { get; }
         public MatchContext Context { get; }
 
+        /// <summary>Handshake cursor (M-HC-3a handler-emit test). Implementing
+        /// <see cref="IHasHandshakePhase"/> lets a test build a <c>FrameDispatchContext</c> over two
+        /// StubParticipants and advance both to <see cref="HandshakePhase.AfterReady"/> so
+        /// <c>BothSidesAfterReady()</c> passes (the PvP relay gate). Harness tests that drive the engine
+        /// directly never read this; it defaults to the pre-handshake state and is harmless to them.</summary>
+        public HandshakePhase Phase { get; set; } = HandshakePhase.AwaitingInitNetwork;
+
         public StubParticipant(long viewerId, MatchContext context)
         {
             ViewerId = viewerId;