fix #381: Options-panel footer needs an opaque backing field

Root cause: a live-DAT probe found retail authors NO backing element
behind the Character/Chat/Config tabs' Apply/Reset/Defaults footer —
each page root has exactly five children (the row ListBox, its
scrollbar, and the three buttons) with zero direct-state media on the
root itself. Scrolled row content therefore bled through visibly
between/behind the three buttons; the bleed-through is a rendering gap
in our own composition, not a missing import.

Fix: new minimal widget UiSolidSpriteFill tiles
RetailChromeSprites.CenterFill (the SAME panel-background sprite the
Options window's own chrome already draws behind everything, not an
invented color) across the footer strip's rect, derived from the three
buttons' own resolved Top/Height and z-ordered strictly behind every
other child so it can never intercept input or occlude the buttons.
Register row AP-205 records the synthesis. Regressed by
OptionsPanelControllerTests.
Bind_SynthesizesOneOpaqueFooterBacking_PerPageWithApplyResetDefaults,
which pins exactly one backing field per page, sized from the live
button rects, z-ordered behind every sibling.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -59,6 +59,67 @@ public sealed class OptionsPanelControllerTests
         Assert.Empty(controller.ConfigPage.Rows);
     }
 
+    /// <summary>Every <see cref="UiSolidSpriteFill"/> under a subtree, in
+    /// build (document) order.</summary>
+    private static List<UiSolidSpriteFill> CollectFooterBackings(UiElement root)
+    {
+        var found = new List<UiSolidSpriteFill>();
+        Walk(root, found);
+        return found;
+
+        static void Walk(UiElement node, List<UiSolidSpriteFill> acc)
+        {
+            if (node is UiSolidSpriteFill fill) acc.Add(fill);
+            foreach (UiElement child in node.Children) Walk(child, acc);
+        }
+    }
+
+    /// <summary>
+    /// #381 regression (2026-08-11, gate 4): before this fix, the
+    /// Character/Chat/Config pages drew their Apply/Reset/Defaults buttons
+    /// directly over the ListBox content with nothing behind them — a live-
+    /// DAT probe found retail authors no backing element either (page root
+    /// has exactly 5 children, zero direct-state media), so this is a
+    /// genuine acdream synthesis (register row AP-205), not a missing
+    /// import. Pins that exactly one opaque field is synthesized per page,
+    /// sized/positioned from the buttons' OWN resolved rects, and z-ordered
+    /// strictly behind everything else on that page (so it can never
+    /// occlude the buttons it backs).
+    /// </summary>
+    [Fact]
+    public void Bind_SynthesizesOneOpaqueFooterBacking_PerPageWithApplyResetDefaults()
+    {
+        ImportedLayout layout = FixtureLoader.LoadOptionsPanelHost();
+        var calls = new List<string>();
+
+        OptionsPanelController? controller = OptionsPanelController.Bind(
+            layout, MakeCallbacks(calls), resolveSprite: _ => (1u, 8, 8));
+
+        Assert.NotNull(controller);
+        // Page slot ids — the SAME literals ConfigOptionsPageController's
+        // own PageSlotElementId/ChatOptionsPageController's own
+        // PageSlotElementId cite, and this file's own TabSwitch tests
+        // already use for Character (0x10000211).
+        foreach (uint pageSlotId in new[] { 0x10000211u, 0x1000050Cu, 0x10000213u })
+        {
+            UiElement pageRoot = UiElement.FindDescendant(controller!.TabPanel, pageSlotId)!;
+            Assert.NotNull(pageRoot);
+            List<UiSolidSpriteFill> backings = CollectFooterBackings(pageRoot);
+            UiSolidSpriteFill backing = Assert.Single(backings);
+
+            Assert.Equal(RetailChromeSprites.CenterFill, backing.SpriteId);
+            Assert.NotNull(backing.SpriteResolve);
+            Assert.True(backing.ClickThrough);
+            Assert.Equal(pageRoot.Width, backing.Width);
+
+            // Drawn BEHIND every sibling already on the page (buttons,
+            // ListBox, scrollbar) — lowest ZOrder wins the painter's-algorithm
+            // back-to-front sort (UiElement.cs's own convention).
+            Assert.All(pageRoot.Children.Where(c => !ReferenceEquals(c, backing)),
+                sibling => Assert.True(backing.ZOrder < sibling.ZOrder));
+        }
+    }
+
     [Fact]
     public void ActivateTabs_SelectsGameplayAsDefault_ButNeverFlushesIt()
     {