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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>