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/BattleSessionState.cs

@@ -56,4 +56,15 @@ internal sealed class BattleSessionState
         foreach (var (idx, cardId, isSelf) in KnownListBuilder.MineAddOps(orderList))
             RecordToken(isSelf == 1 ? from : other, idx, cardId);
     }
+
+    /// <summary>Mine + record choice/Discover-token picks (<see cref="KnownListBuilder.MineChoicePicks"/>)
+    /// into the correct side's map, by the same <c>isSelf</c> routing as <see cref="RecordTokensFrom"/>.
+    /// The chosen cardId rides the generating send's <c>keyAction.selectCard</c> (not the orderList add
+    /// op, which carries candidates only); recorded regardless of the choice's <c>open</c> visibility —
+    /// an unplayed idx is never queried, so a stray record is harmless.</summary>
+    public void RecordChoicePicksFrom(IBattleParticipant from, IBattleParticipant other, object? orderList, object? keyAction)
+    {
+        foreach (var (idx, cardId, isSelf) in KnownListBuilder.MineChoicePicks(orderList, keyAction))
+            RecordToken(isSelf == 1 ? from : other, idx, cardId);
+    }
 }