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.BattleNode/Protocol/Bodies/PlayActionsBroadcastBody.cs

@@ -36,15 +36,19 @@ public sealed record SelectCardEntry(
     [property: JsonPropertyName("open")]
     [property: JsonConverter(typeof(JsonNumberEnumConverter<ChoiceVisibility>))] ChoiceVisibility Open);
 
-/// <summary>One revealed card in a <c>knownList</c>. Vanilla slice fills cardId from the sender's
-/// deck map and leaves spellboost 0 / attachTarget "" (cost/clan/tribe deferred to the card-master
-/// port — the receiver re-derives them from cardId).</summary>
+/// <summary>One revealed card in a <c>knownList</c>. <c>cardId</c> from the sender's deck map; <c>cost</c>
+/// is the ENGINE-RESOLVED play-time cost (M-HC-3a) — the discounted cost the headless engine actually
+/// charged (spellboost + board modifiers folded in by construction), emitted on EVERY entry (prod sends
+/// cost 45/45 in captures, so it is NOT omitted). <c>spellboost</c> still carries the count for now
+/// (Task 6 retires that bookkeeping once cost is engine-sourced everywhere). attachTarget stays "";
+/// clan/tribe remain deferred (receiver re-derives them from cardId).</summary>
 public sealed record KnownCardEntry(
     [property: JsonPropertyName("idx")] int Idx,
     [property: JsonPropertyName("cardId")] long CardId,
     [property: JsonPropertyName("to")] int To,
     [property: JsonPropertyName("spellboost")] int Spellboost,
-    [property: JsonPropertyName("attachTarget")] string AttachTarget);
+    [property: JsonPropertyName("attachTarget")] string AttachTarget,
+    [property: JsonPropertyName("cost")] int Cost);
 
 /// <summary>Renamed <c>targetList</c> entry. <c>isSelf</c> is actor-relative and passes through
 /// verbatim — no perspective flip (bullet-3 audit F2).</summary>