fix(ui): D.2b-B — inventory panels render over the backdrop (#145 continuation) + captions

Visual verification surfaced two render bugs (controller LOGIC was already correct):

1. BACKDROP WASH-OUT (the big one — #145 continuation). The mounted backpack/
   3D-items panels inherited their sub-window root's ZLevel 1000 via the merge's
   zero-wins-base rule. The #145 ZOrder fold (ReadOrder − ZLevel·10000) turned 1000
   into ZOrder ≈ −10,000,000 — sinking the panels BEHIND the frame's Alphablend
   backdrop (ZLevel 100 → ≈ −1,000,000). The backdrop then overpainted the panels'
   captions/burden-meter/cells (the paperdoll root is ZLevel 0 so it escaped, which
   is why the previous session thought #145 was done). Fix: the sub-window mount now
   keeps each slot's OWN frame ZLevel, so panels sit in front of the backdrop.
   Root-caused via a one-shot sprite-segment-order dump (backdrop was painting after
   the panel content) + a live ZLevel probe.

2. CAPTIONS. The caption elements resolve to UiText; driving a nested child UiText
   didn't paint. AttachCaption now drives the host UiText directly.

Locked by InventoryFrameImportProbe (real-dat smoke: asserts each mounted panel's
ZOrder > the backdrop's). Visually confirmed by the user: dark backdrop behind,
Burden 17% + vertical bar + Contents-of-Backpack + full item grid all visible.
Build + App(532)/Core(1526) tests green.

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

tests/AcDream.App.Tests/UI/Layout/InventoryFrameImportProbe.cs

@@ -0,0 +1,68 @@
+using System;
+using System.IO;
+using AcDream.App.UI.Layout;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using Xunit;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+/// <summary>
+/// Real-dat smoke test for the gmInventoryUI frame (0x21000023) import + the sub-window
+/// mount. Locks the #145-continuation fix: the mount keeps each mounted panel slot's OWN
+/// frame ZLevel instead of inheriting the sub-window root's ZLevel 1000. Before the fix the
+/// merge's zero-wins-base rule gave the slots ZLevel 1000 → the #145 ZOrder fold
+/// (ReadOrder − ZLevel·10000) sank them to ≈ −10,000,000, BEHIND the frame's Alphablend
+/// backdrop (ZLevel 100 → ≈ −1,000,000), and the backdrop washed out the panels' captions,
+/// burden meter, and item cells. Skips when the live dat directory is absent (CI).
+/// </summary>
+public class InventoryFrameImportProbe
+{
+    private const uint Frame          = 0x21000023u;
+    private const uint Backdrop       = 0x100001D0u;   // full-window Alphablend backdrop (ZLevel 100)
+    private const uint BackpackPanel  = 0x100001CEu;   // mounted gmBackpackUI slot
+    private const uint ItemsPanel     = 0x100001CFu;   // mounted gm3DItemsUI slot
+    private const uint PaperdollPanel = 0x100001CDu;   // mounted gmPaperDollUI slot
+    private const uint BurdenMeter    = 0x100001D9u;   // backpack content (proves the mount attached it)
+    private const uint ContentsGrid   = 0x100001C6u;   // 3D-items content
+
+    private static string? DatDir()
+    {
+        var d = Environment.GetEnvironmentVariable("ACDREAM_DAT_DIR")
+            ?? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
+                            "Documents", "Asheron's Call");
+        return Directory.Exists(d) ? d : null;
+    }
+
+    [Fact]
+    public void Mounted_panels_sit_in_front_of_the_backdrop()
+    {
+        var datDir = DatDir();
+        if (datDir is null) return;   // CI: no live dat — skip (this is a smoke test)
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var layout = LayoutImporter.Import(dats, Frame, _ => (0u, 0, 0), null);
+        Assert.NotNull(layout);
+
+        var backdrop = layout!.FindElement(Backdrop);
+        Assert.NotNull(backdrop);   // the full-window backdrop is a direct child of the frame
+
+        // The sub-window mount brings each panel's content into the by-id index.
+        Assert.NotNull(layout.FindElement(BurdenMeter));    // backpack burden meter
+        Assert.NotNull(layout.FindElement(ContentsGrid));   // 3D-items contents grid
+
+        // The fix: every mounted panel slot must draw IN FRONT of the backdrop
+        // (higher ZOrder = painted later = on top), so the backdrop sits behind the content.
+        foreach (var (id, name) in new[]
+        {
+            (BackpackPanel, "backpack"), (ItemsPanel, "3D-items"), (PaperdollPanel, "paperdoll"),
+        })
+        {
+            var panel = layout.FindElement(id);
+            Assert.True(panel is not null, $"{name} panel 0x{id:X8} missing from the imported tree");
+            Assert.True(panel!.ZOrder > backdrop!.ZOrder,
+                $"{name} panel ZOrder {panel.ZOrder} must be > backdrop ZOrder {backdrop.ZOrder} " +
+                "(else the Alphablend backdrop overpaints/washes out the panel content)");
+        }
+    }
+}