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>();
     }