Swap Meet trades treasures — a carried treasure is a carried item

The trade grammar gains a 'treasure' token (never present in stored
ledgers, so no gate needed): a dagger can buy back the treasure in a
thief's arms, or two armfuls of gold can change hands outright. The
one-treasure carry limit and WEAKNESS both hold in a trade as at a
grab — a one-way treasure needs an open slot and an able back. The
picker offers each side's carried treasure alongside displayed items,
withholding theirs when your own arms are already full.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0138A8CjeQRpvzKxuMfz1Bqc

packages/engine/src/game.ts

@@ -2155,20 +2155,58 @@ const CARD_EFFECTS: Record<string, AttackEffect | NeutralEffect | CounterEffect>
       const [mineId, theirsId] = (ctx.stack.params!.cardId ?? "").split(";");
       // "Swap any two carried items": every movable object trades — daggers,
       // rocks, wands, stones (rules rev 26; earlier games matched only
-      // object-typed cards and replay so).
+      // object-typed cards and replay so) — and a carried TREASURE is an
+      // item too, named by the "treasure" token (never in older ledgers).
       const tradable = (id: string) =>
         (ctx.state.config.deckRev ?? 1) >= 26
           ? isMovableObject(id)
           : cardDef(id).cardType === "object";
-      const mine = ctx.attacker.hand.findIndex((c) => c.cardId === mineId && tradable(c.cardId));
-      const theirs = ctx.defender.hand.findIndex((c) => c.cardId === theirsId && tradable(c.cardId));
-      if (mine === -1 || theirs === -1) return;
-      const [a] = ctx.attacker.hand.splice(mine, 1);
-      const [b] = ctx.defender.hand.splice(theirs, 1);
-      ctx.attacker.hand.push(b!);
-      ctx.defender.hand.push(a!);
-      ctx.attacker.displayed = ctx.attacker.displayed.filter((id) => id !== a!.instanceId);
-      ctx.defender.displayed = ctx.defender.displayed.filter((id) => id !== b!.instanceId);
+      type TradeSide = { kind: "card"; idx: number } | { kind: "treasure" };
+      const side = (p: PlayerState, id: string): TradeSide | null => {
+        if (id === "treasure") return p.carriedTreasureId ? { kind: "treasure" } : null;
+        const idx = p.hand.findIndex((c) => c.cardId === id && tradable(c.cardId));
+        return idx === -1 ? null : { kind: "card", idx };
+      };
+      const mine = side(ctx.attacker, mineId ?? "");
+      const theirs = side(ctx.defender, theirsId ?? "");
+      if (!mine || !theirs) return;
+      // A one-way treasure needs an open carry slot and an able back:
+      // "you are too weak to carry treasure" holds in a trade as at a grab.
+      const weak = (p: PlayerState) => sustainedOn(ctx.state, p.id, "weakness").length > 0;
+      if (mine.kind === "treasure" && theirs.kind !== "treasure" &&
+          (ctx.defender.carriedTreasureId != null || weak(ctx.defender))) return;
+      if (theirs.kind === "treasure" && mine.kind !== "treasure" &&
+          (ctx.attacker.carriedTreasureId != null || weak(ctx.attacker))) return;
+      const giveTreasure = (from: PlayerState, to: PlayerState) => {
+        const t = ctx.state.treasures.find((t) => t.id === from.carriedTreasureId)!;
+        from.carriedTreasureId = null;
+        t.carriedBy = to.id;
+        return t;
+      };
+      const takeCard = (p: PlayerState, s: Extract<TradeSide, { kind: "card" }>) => {
+        const [card] = p.hand.splice(s.idx, 1);
+        p.displayed = p.displayed.filter((id) => id !== card!.instanceId);
+        return card!;
+      };
+      if (mine.kind === "treasure" && theirs.kind === "treasure") {
+        const tA = giveTreasure(ctx.attacker, ctx.defender);
+        const tB = giveTreasure(ctx.defender, ctx.attacker);
+        ctx.defender.carriedTreasureId = tA.id;
+        ctx.attacker.carriedTreasureId = tB.id;
+      } else if (mine.kind === "treasure") {
+        const t = giveTreasure(ctx.attacker, ctx.defender);
+        ctx.defender.carriedTreasureId = t.id;
+        ctx.attacker.hand.push(takeCard(ctx.defender, theirs as Extract<TradeSide, { kind: "card" }>));
+      } else if (theirs.kind === "treasure") {
+        const t = giveTreasure(ctx.defender, ctx.attacker);
+        ctx.attacker.carriedTreasureId = t.id;
+        ctx.defender.hand.push(takeCard(ctx.attacker, mine));
+      } else {
+        const a = takeCard(ctx.attacker, mine);
+        const b = takeCard(ctx.defender, theirs);
+        ctx.attacker.hand.push(b);
+        ctx.defender.hand.push(a);
+      }
       ctx.events.push({ type: "itemsSwapped", a: ctx.attacker.id, b: ctx.defender.id });
     },
   },