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.App.Tests/UI/Layout/ItemCooldownUiControllerTests.cs

@@ -0,0 +1,62 @@
+using AcDream.App.UI;
+using AcDream.App.UI.Layout;
+using AcDream.Core.Items;
+using AcDream.Core.Spells;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+public sealed class ItemCooldownUiControllerTests
+{
+    [Fact]
+    public void Shared_group_updates_existing_and_future_UIItems_from_one_clock()
+    {
+        var objects = new ClientObjectTable();
+        objects.AddOrUpdate(new ClientObject
+        {
+            ObjectId = 0x5001u,
+            CooldownId = 42u,
+            CooldownDuration = 30d,
+        });
+
+        var spellbook = new Spellbook();
+        spellbook.OnEnchantmentAdded(new ActiveEnchantmentRecord(
+            SpellId: 0x802Au,
+            LayerId: 1u,
+            Duration: 30d,
+            CasterGuid: 0u,
+            Bucket: Spellbook.CooldownBucket,
+            StartTime: 100d));
+
+        double now = 112.5d;
+        var root = new UiPanel();
+        var list = new UiItemList();
+        list.Cell.SetItem(0x5001u, 99u);
+        root.AddChild(list);
+        uint[] sprites = Enumerable.Range(1, 10)
+            .Select(index => 0x06000000u + (uint)index)
+            .ToArray();
+
+        ItemCooldownUiController controller = ItemCooldownUiController.Bind(
+            root,
+            spellbook,
+            objects,
+            () => now,
+            new ItemCooldownAssets(sprites));
+
+        Assert.Equal(sprites[5], list.Cell.ActiveCooldownSprite());
+
+        var future = new UiItemSlot();
+        future.SetItem(0x5001u, 100u);
+        list.AddItem(future);
+        Assert.Equal(sprites[5], future.ActiveCooldownSprite());
+
+        now = 129.999d;
+        controller.Tick();
+        Assert.Equal(sprites[0], list.Cell.ActiveCooldownSprite());
+
+        now = 130d;
+        controller.Tick();
+        Assert.Equal(0u, list.Cell.ActiveCooldownSprite());
+        Assert.Equal(0, spellbook.ActiveCount);
+    }
+}