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>

tests/AcDream.Core.Tests/Spells/SpellbookTests.cs

@@ -160,6 +160,87 @@ public sealed class SpellbookTests
         Assert.Equal(0, book.ActiveCount);
     }
 
+    [Fact]
+    public void OnCooldown_removes_first_expired_match_then_reveals_next_record()
+    {
+        var book = new Spellbook();
+        var expired = new ActiveEnchantmentRecord(
+            SpellId: 0x802Au,
+            LayerId: 1u,
+            Duration: 5d,
+            CasterGuid: 0u,
+            Bucket: Spellbook.CooldownBucket,
+            StartTime: 0d);
+        var active = expired with
+        {
+            LayerId = 2u,
+            Duration = 30d,
+            StartTime = 10d,
+        };
+        book.ReplaceManifest(
+            new Dictionary<uint, float>(),
+            [expired, active],
+            Array.Empty<IReadOnlyList<uint>>(),
+            Array.Empty<(uint Id, uint Amount)>(),
+            0u);
+        ActiveEnchantmentRecord? removed = null;
+        book.EnchantmentRemoved += record => removed = record;
+
+        Assert.False(book.OnCooldown(
+            0x2Au,
+            currentTime: 20d,
+            out double expiredRemaining));
+        Assert.Equal(-15d, expiredRemaining);
+        Assert.Equal(expired, removed);
+        Assert.Equal(1, book.ActiveCount);
+
+        Assert.True(book.OnCooldown(
+            0x2Au,
+            currentTime: 20d,
+            out double activeRemaining));
+        Assert.Equal(20d, activeRemaining);
+    }
+
+    [Fact]
+    public void OnCooldown_ignores_same_spell_outside_cooldown_registry()
+    {
+        var book = new Spellbook();
+        book.OnEnchantmentAdded(new ActiveEnchantmentRecord(
+            SpellId: 0x802Au,
+            LayerId: 1u,
+            Duration: 30d,
+            CasterGuid: 0u,
+            Bucket: 1u,
+            StartTime: 10d));
+
+        Assert.False(book.OnCooldown(
+            0x2Au,
+            currentTime: 20d,
+            out double remaining));
+        Assert.Equal(0d, remaining);
+        Assert.Equal(1, book.ActiveCount);
+    }
+
+    [Fact]
+    public void OnCooldown_zero_group_is_inactive_and_does_not_remove()
+    {
+        var book = new Spellbook();
+        book.OnEnchantmentAdded(new ActiveEnchantmentRecord(
+            SpellId: 0x8000u,
+            LayerId: 1u,
+            Duration: 30d,
+            CasterGuid: 0u,
+            Bucket: Spellbook.CooldownBucket,
+            StartTime: 10d));
+
+        Assert.False(book.OnCooldown(
+            cooldownId: 0u,
+            currentTime: 20d,
+            out double remaining));
+        Assert.Equal(0d, remaining);
+        Assert.Equal(1, book.ActiveCount);
+    }
+
     [Fact]
     public void OnPurgeAll_RemovesOnlyFiniteMultAndAddEnchantments()
     {