perf(ui): scope cooldown heartbeat to visible items

src/AcDream.Core/Spells/Spellbook.cs

@@ -156,6 +156,11 @@ public sealed class Spellbook
 
     public int LearnedCount => _learnedSpells.Count;
     public int ActiveCount  => _activeById.Count;
+    public bool HasCooldownEnchantments =>
+        _enchantmentOrderByBucket.TryGetValue(
+            CooldownBucket,
+            out List<uint>? order)
+        && order.Count != 0;
 
     public bool Knows(uint spellId) => _learnedSpells.ContainsKey(spellId);