Initial pass at unicorn pack.

internal/ai/shop.go

@@ -52,6 +52,20 @@ func (cx *ctx) applyTemplateShopEffects(deck []game.Card, c game.Card, trigger g
 			for range best {
 				deck = append(deck, cx.simApple())
 			}
+		case game.ActionRevealTierForApples:
+			// Quetzalcoatl: a fixed apple reward if a low-tier pet can be revealed.
+			eligible := false
+			for _, d := range deck {
+				if d.IsPet() && d.ID != c.ID && (e.Cap <= 0 || d.Tier <= e.Cap) {
+					eligible = true
+					break
+				}
+			}
+			if eligible {
+				for range max(e.Count, 1) {
+					deck = append(deck, cx.simApple())
+				}
+			}
 		case game.ActionDoubleApples:
 			apples := 0
 			for _, dc := range deck {
@@ -87,6 +101,39 @@ func (cx *ctx) applyTemplateShopEffects(deck []game.Card, c game.Card, trigger g
 			if i := slices.IndexFunc(deck, func(d game.Card) bool { return d.ID == c.ID }); i >= 0 {
 				deck = slices.Delete(deck, i, i+1)
 			}
+		case game.ActionUpgradeNextTier:
+			// Unicorn pack (Water of Youth): discard this food and the lowest-value
+			// pet, then gain a sampled top card of the next tier for free.
+			nextTier := cx.v.Round + 1
+			if nextTier > game.MaxRounds {
+				continue
+			}
+			if i := slices.IndexFunc(deck, func(d game.Card) bool { return d.ID == c.ID }); i >= 0 {
+				deck = slices.Delete(deck, i, i+1)
+			}
+			worst, worstVal := -1, 0.0
+			for i, d := range deck {
+				if !d.IsPet() {
+					continue
+				}
+				if v := keepValue(d); worst < 0 || v < worstVal {
+					worst, worstVal = i, v
+				}
+			}
+			if worst < 0 {
+				continue // no pet to sacrifice; the food is wasted
+			}
+			deck = slices.Delete(deck, worst, worst+1)
+			pool := cx.unseenPool(nextTier)
+			if len(pool) == 0 {
+				pool = game.TierContentsForPack(cx.v.Pack, nextTier)
+			}
+			if len(pool) > 0 {
+				rc := pool[rand.IntN(len(pool))]
+				rc.ID = cx.nextSimID()
+				deck = append(deck, rc)
+				deck = cx.applyTemplateShopEffects(deck, rc, game.TriggerBuy)
+			}
 		}
 	}
 	return deck