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/Sessions/KnownListBuilderTests.cs

@@ -82,6 +82,18 @@ public class KnownListBuilderTests
         Assert.That(entry.To, Is.EqualTo(20));
         Assert.That(entry.Spellboost, Is.EqualTo(0));
         Assert.That(entry.AttachTarget, Is.EqualTo(""));
+        Assert.That(entry.Cost, Is.EqualTo(0), "cost defaults to 0 when the caller passes none");
+    }
+
+    [Test]
+    public void BuildPlayedCard_emits_engine_resolved_cost_passed_by_caller()
+    {
+        // M-HC-3a: the handler reads the engine-resolved play-time cost and passes it in; BuildPlayedCard
+        // lands it on the entry verbatim. (A wrong cost yields a different field — non-vacuity.)
+        var deckMap = new Dictionary<int, long> { [3] = 101314020L };
+        var entry = KnownListBuilder.BuildPlayedCard(deckMap, playIdx: 3, orderList: OrderListMove(3, 10, 20), spellboostMap: null, cost: 3);
+        Assert.That(entry, Is.Not.Null);
+        Assert.That(entry!.Cost, Is.EqualTo(3));
     }
 
     [Test]