feat(battle-node): reveal choice/Discover tokens to opponent

Choice/Discover-into-hand fanfares add a candidates-only token to hand; the
chosen cardId rides keyAction.selectCard on the generating play, not the
orderList add op. Record idx->chosenCardId at generation (candidate-membership
join) so the later play reveals the real identity via the existing
BuildPlayedCard path; forward {type,cardId} to the opponent and strip
selectCard for hidden (open:0) picks (pass through for open:1, provisional).

- KnownListBuilder.MineChoicePicks + StripKeyActionForOpponent (pure)
- BattleSessionState.RecordChoicePicksFrom (reuses IdxToCardId, no new state)
- PlayActionsBroadcastBody.keyAction + KeyActionEntry/SelectCardEntry
- PlayActionsHandler wires both; EchoHandler unchanged (picks ride the send)

Tests (TDD red->green): 8 KnownListBuilder + 2 dispatch + 2 conformance
(shape-locked to tk2_regular L151 generation / L193 reveal). Full suite 976/0.

Spec: docs/superpowers/specs/2026-06-04-battle-node-choice-token-reveal-design.md

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

SVSim.BattleNode/Sessions/Dispatch/Handlers/EchoHandler.cs

@@ -16,6 +16,9 @@ internal sealed class EchoHandler : IFrameHandler
         {
             var orderList = (ctx.Env.Body as RawBody)?.Entries.GetValueOrDefault("orderList");
             ctx.State.RecordTokensFrom(ctx.From, ctx.Other, orderList);
+            // No RecordChoicePicksFrom here: choice picks ride keyAction.selectCard on the generating
+            // SEND, not the receiver's Echo (Echo carries orderList only) — the pick is already
+            // recorded by PlayActionsHandler. MineChoicePicks(orderList, null) would yield nothing.
         }
         return Array.Empty<DispatchRoute>();
     }

SVSim.BattleNode/Sessions/Dispatch/Handlers/PlayActionsHandler.cs

@@ -3,11 +3,12 @@ using SVSim.BattleNode.Protocol.Bodies;
 
 namespace SVSim.BattleNode.Sessions.Dispatch.Handlers;
 
-/// <summary>PvP PlayActions translator (vanilla deck-card slice). Synthesizes the opponent-facing
-/// knownList from the sender's idx->cardId map + the orderList move op, renames targetList ->
-/// oppoTargetList, drops orderList, consumes keyAction.
-/// Token plays resolve their cardId from add ops mined on earlier frames; an un-generated token
-/// idx still degrades to {playIdx,type} (no knownList). Bot drop (no rule).</summary>
+/// <summary>PvP PlayActions translator. Synthesizes the opponent-facing knownList from the sender's
+/// idx->cardId map + the orderList move op, renames targetList -> oppoTargetList, drops orderList,
+/// and forwards a stripped keyAction for choice/Discover plays ({type,cardId}; selectCard dropped
+/// for a hidden open:0 pick). Token plays resolve their cardId from add ops (concrete tokens) or
+/// keyAction.selectCard (choice picks) mined on earlier frames; an un-generated token idx still
+/// degrades to {playIdx,type} (no knownList). Bot drop (no rule).</summary>
 internal sealed class PlayActionsHandler : IFrameHandler
 {
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
@@ -20,12 +21,17 @@ internal sealed class PlayActionsHandler : IFrameHandler
         var type = (int)KnownListBuilder.AsLong(entries.GetValueOrDefault("type"));
 
         var orderList = entries.GetValueOrDefault("orderList");
+        var keyAction = entries.GetValueOrDefault("keyAction");
 
         // Mine generated-token identities from this frame's add ops into the right side's idx->cardId
         // map (isSelf:1 → sender; isSelf:0 → opponent, a cross-side gift), so a token played in a LATER
         // frame resolves its cardId — by whichever side ends up playing it (bullet-3 audit F1).
         ctx.State.RecordTokensFrom(ctx.From, ctx.Other, orderList);
 
+        // Choice/Discover-into-hand: the chosen cardId rides keyAction.selectCard (the orderList's
+        // choiceAdd carries candidates only). Record idx->chosenCardId now so the later play reveals it.
+        ctx.State.RecordChoicePicksFrom(ctx.From, ctx.Other, orderList, keyAction);
+
         var deckMap = ctx.State.GetOrSeedDeckMap(ctx.From);
         var played = KnownListBuilder.BuildPlayedCard(deckMap, playIdx, orderList);
         var oppoTargets = KnownListBuilder.RenameTargets(entries.GetValueOrDefault("targetList"));
@@ -34,7 +40,10 @@ internal sealed class PlayActionsHandler : IFrameHandler
             PlayIdx: playIdx,
             Type: type,
             KnownList: played is null ? null : new[] { played },
-            OppoTargetList: oppoTargets);
+            OppoTargetList: oppoTargets,
+            // {type,cardId} forwarded so the opponent renders the choice token; selectCard dropped
+            // when open==0 (hidden draw-to-hand pick). Null for a vanilla play (no keyAction).
+            KeyAction: KnownListBuilder.StripKeyActionForOpponent(keyAction));
 
         var frame = ctx.Env with { Body = body };
         return new[] { new DispatchRoute(ctx.Other, frame, false) };