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/Items/ItemCooldownDisplay.cs

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+namespace AcDream.Core.Items;
+
+/// <summary>
+/// Retail item-cooldown projection shared by inventory, equipment, and shortcut
+/// UIItems.
+///
+/// <para>
+/// Port of <c>UIElement_UIItem::UpdateCooldownDisplay @ 0x004E1E20</c>.
+/// The remaining fraction selects one of ten authored radial overlays.
+/// </para>
+/// </summary>
+public static class ItemCooldownDisplay
+{
+    /// <summary>
+    /// Retail's exact ten-step display formula:
+    /// <c>trunc((remaining / itemDuration) * 10 + 1)</c>. Results 1..9 select
+    /// the 10%..90% art, while 10 or above selects the 100% art.
+    /// </summary>
+    public static int GetOverlayStep(double itemDuration, double remaining)
+    {
+        if (!(itemDuration > 0d) || !(remaining > 0d))
+            return 0;
+
+        double step = Math.Truncate((remaining / itemDuration) * 10d + 1d);
+        if (!(step >= 1d))
+            return 0;
+        if (step >= 10d)
+            return 10;
+        return (int)step;
+    }
+}