fix(ui): #145 — importer honors ZLevel so the backdrop sits behind panels

The factory mapped ZOrder from ReadOrder only, so the gmInventoryUI
full-window backdrop (0x100001D0, ReadOrder 4) painted over the nested
panels (ReadOrder 1-3). Wire ElementDesc.ZLevel through ElementInfo /
ToInfo / Merge and fold it into ZOrder = ReadOrder - ZLevel*10000 (higher
ZLevel = further back, ReadOrder the within-layer tiebreaker). Vitals
(all ZLevel 0) are unchanged; chat (ZLevel 900) + toolbar (1,2) shift to
their dat layering — verify visually.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/Layout/ElementReader.cs

@@ -39,6 +39,12 @@ public sealed class ElementInfo
     /// <summary>Draw order within the parent (lower = drawn first / behind).</summary>
     public uint ReadOrder;
 
+    /// <summary>Layer level from the dat (<c>ElementDesc.ZLevel</c>). Higher = drawn further
+    /// BACK (a full-window backdrop at ZLevel 100 sits behind ZLevel-0 panels). The factory
+    /// folds it into <see cref="UiElement.ZOrder"/> so ZLevel dominates and ReadOrder is the
+    /// within-layer tiebreaker. Issue #145 (vitals are all ZLevel 0, so they're unaffected).</summary>
+    public uint ZLevel;
+
     /// <summary>
     /// Font dat object id inherited from the base element's <c>Properties[0x1A]</c>
     /// (<c>ArrayBaseProperty → DataIdBaseProperty</c>). 0 = none / not inherited.
@@ -152,6 +158,7 @@ public static class ElementReader
             Right     = derived.Right,
             Bottom    = derived.Bottom,
             ReadOrder = derived.ReadOrder,
+            ZLevel    = derived.ZLevel != 0 ? derived.ZLevel : base_.ZLevel,
             FontDid          = derived.FontDid   != 0 ? derived.FontDid   : base_.FontDid,
             // DefaultStateName: derived wins if set; otherwise inherit the base's default.
             DefaultStateName = !string.IsNullOrEmpty(derived.DefaultStateName) ? derived.DefaultStateName : base_.DefaultStateName,