feat(items): port retail shared cooldown overlays

Preserve public shared-cooldown metadata, resolve the authoritative cooldown enchantment with retail expiry semantics, and project the exact ten DAT-authored radial steps through the shared retained item-slot architecture.

Co-authored-by: Codex <codex@openai.com>

src/AcDream.Core/Spells/Spellbook.cs

@@ -17,6 +17,9 @@ namespace AcDream.Core.Spells;
 /// </summary>
 public sealed class Spellbook
 {
+    public const uint CooldownSpellOffset = 0x8000u;
+    public const uint CooldownBucket = 8u;
+
     private readonly Dictionary<uint, float> _learnedSpells = new();
     private readonly Dictionary<uint, ActiveEnchantmentRecord> _activeById = new();
     private readonly Dictionary<uint, List<uint>> _enchantmentOrderByBucket = new();
@@ -240,6 +243,46 @@ public sealed class Spellbook
         if (changed) NotifyEnchantmentsChanged();
     }
 
+    /// <summary>
+    /// Retail <c>CEnchantmentRegistry::OnCooldown @ 0x005943C0</c>.
+    /// Scans the cooldown list head-to-tail, stops at the first shared-group
+    /// match, and removes that node when its monotonic lifetime has expired.
+    /// </summary>
+    public bool OnCooldown(
+        uint cooldownId,
+        double currentTime,
+        out double remaining)
+    {
+        remaining = 0d;
+        if (cooldownId == 0u
+            || !_enchantmentOrderByBucket.TryGetValue(
+                CooldownBucket,
+                out List<uint>? order))
+            return false;
+
+        uint cooldownSpellId = unchecked(cooldownId + CooldownSpellOffset);
+        for (int index = 0; index < order.Count; index++)
+        {
+            uint identity = order[index];
+            if (!_activeById.TryGetValue(
+                    identity,
+                    out ActiveEnchantmentRecord record)
+                || (record.SpellId & 0xFFFFu) != cooldownSpellId)
+                continue;
+
+            remaining = record.Duration + record.StartTime - currentTime;
+            if (remaining > 0d)
+                return true;
+
+            RemoveActiveEnchantment(identity, out _);
+            EnchantmentRemoved?.Invoke(record);
+            NotifyEnchantmentsChanged();
+            return false;
+        }
+
+        return false;
+    }
+
     /// <summary>
     /// Retail EnchantmentRegistry::PurgeEnchantments (0x00594DE0): purge only
     /// finite-duration multiplicative/additive enchantments. Vitae, cooldowns,