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/Items/ItemCooldownDisplayTests.cs

@@ -0,0 +1,22 @@
+using AcDream.Core.Items;
+namespace AcDream.Core.Tests.Items;
+
+public sealed class ItemCooldownDisplayTests
+{
+    [Theory]
+    [InlineData(10.0, 10.0, 10)]
+    [InlineData(10.0, 9.0, 10)]
+    [InlineData(10.0, 8.999, 9)]
+    [InlineData(10.0, 5.0, 6)]
+    [InlineData(10.0, 0.001, 1)]
+    [InlineData(10.0, 0.0, 0)]
+    [InlineData(0.0, 5.0, 0)]
+    public void GetOverlayStep_matches_retail_truncation(
+        double duration,
+        double remaining,
+        int expected)
+        => Assert.Equal(
+            expected,
+            ItemCooldownDisplay.GetOverlayStep(duration, remaining));
+
+}