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>

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

@@ -169,9 +169,18 @@ public sealed class OptionsPanelController : IRetainedPanelController
     /// import in this codebase (<see cref="CharacterController"/>,
     /// <see cref="RetailDialogFactory"/>'s dialog catalog).
     /// </summary>
+    /// <param name="resolveSprite">#381: RenderSurface id → (GL tex handle,
+    /// pixel width, pixel height) — wired onto each page's synthesized
+    /// footer backing field (<see cref="UiSolidSpriteFill"/>). Null leaves
+    /// the field present but drawing nothing (matching every other
+    /// null-safe sprite resolver in this codebase) — the fixture/
+    /// conformance callers that never exercise the visual layer.</param>
     /// <returns>Null if <paramref name="layout"/>'s root did not build as a
     /// <see cref="UiTabPanel"/> (a missing/malformed LayoutDesc).</returns>
-    public static OptionsPanelController? Bind(ImportedLayout layout, Callbacks callbacks)
+    public static OptionsPanelController? Bind(
+        ImportedLayout layout,
+        Callbacks callbacks,
+        Func<uint, (uint tex, int w, int h)>? resolveSprite = null)
     {
         ArgumentNullException.ThrowIfNull(layout);
         ArgumentNullException.ThrowIfNull(callbacks);
@@ -223,7 +232,16 @@ public sealed class OptionsPanelController : IRetainedPanelController
 
             UiButton? apply = BindPageButton(pageRoot, ApplyButtonId, page.Apply);
             UiButton? reset = BindPageButton(pageRoot, ResetButtonId, page.Reset);
-            BindPageButton(pageRoot, DefaultsButtonId, page.Defaults);
+            UiButton? defaults = BindPageButton(pageRoot, DefaultsButtonId, page.Defaults);
+
+            // #381 (2026-08-11, gate 4): the opaque backing field behind the
+            // footer strip — see UiSolidSpriteFill's own doc for the
+            // live-DAT probe proving retail authors no such element (the
+            // page root has exactly 5 children, zero direct-state media).
+            // Sized from the THREE BUTTONS' OWN resolved rects (not a
+            // hardcoded literal) so it tracks whatever the authored layout
+            // actually placed them at.
+            AddFooterBacking(pageRoot, apply, reset, defaults, resolveSprite);
 
             // MUST-FIX 2 (OP4 review-fix round, 2026-08-11): retail
             // PlayerOptionPage::OnOptionChanged @0x004F27D0 — Apply/Reset
@@ -264,6 +282,53 @@ public sealed class OptionsPanelController : IRetainedPanelController
         return null;
     }
 
+    /// <summary>Draws behind everything else already added to
+    /// <paramref name="pageRoot"/> — see <see cref="UiSolidSpriteFill"/>'s
+    /// own doc for why this element exists at all (AP-205, synthesized,
+    /// no authored counterpart).</summary>
+    private const int FooterBackingZOrder = int.MinValue / 2;
+
+    /// <summary>
+    /// #381: inserts the opaque, borderless backing field behind
+    /// Apply/Reset/Defaults, deriving its Y/height from whichever of the
+    /// three physical buttons resolved (all three share the SAME authored
+    /// Y=564/Height=32, research doc §3.1 — any one is sufficient). Skipped
+    /// (not an error — logged) only when NONE resolved; a page missing all
+    /// three already logged its own per-button warnings above.
+    /// </summary>
+    private static void AddFooterBacking(
+        UiElement pageRoot,
+        UiButton? apply,
+        UiButton? reset,
+        UiButton? defaults,
+        Func<uint, (uint tex, int w, int h)>? resolveSprite)
+    {
+        UiButton? first = apply ?? reset ?? defaults;
+        if (first is null)
+        {
+            Console.WriteLine(
+                $"[D.2b] OptionsPanelController: page 0x{pageRoot.DatElementId:X8} has no "
+                + "resolved Apply/Reset/Defaults buttons — footer backing field skipped.");
+            return;
+        }
+
+        // Span the FULL authored page width (not just the button union) —
+        // retail's own three buttons don't reach the page's right edge, but
+        // the reported bleed-through is content scrolling behind the WHOLE
+        // footer strip, not just the button rects themselves.
+        var backing = new UiSolidSpriteFill
+        {
+            SpriteId = RetailChromeSprites.CenterFill,
+            SpriteResolve = resolveSprite,
+            Left = 0f,
+            Top = first.Top,
+            Width = pageRoot.Width,
+            Height = first.Height,
+            ZOrder = FooterBackingZOrder,
+        };
+        pageRoot.AddChild(backing);
+    }
+
     /// <summary>
     /// Activates the tab-switching behavior (idempotent — safe even if
     /// already active). Must run AFTER <see cref="Bind"/> so this