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/UiSolidSpriteFill.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Numerics;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// #381 (Campaign OP gate 4, 2026-08-11): a minimal, purely-synthesized
+/// leaf that tiles ONE opaque sprite across its own rect — nothing more.
+/// Used for the Apply/Reset/Defaults footer's opaque backing field: a
+/// live-DAT probe found the Character/Chat/Config page roots author
+/// EXACTLY five children each (the row ListBox, its scrollbar, and the
+/// three physical buttons) with zero direct-state media on the root itself
+/// — retail authors NO backdrop element behind the footer strip at all, so
+/// scrolled row content bled through between/behind the three buttons.
+///
+/// <para>
+/// This is a genuine synthesis, not a retail port — register row AP-205
+/// records it. The fill sprite is <see cref="RetailChromeSprites.CenterFill"/>,
+/// the SAME authored panel-background media the whole Options window's own
+/// <see cref="UiNineSlicePanel"/> chrome already tiles behind everything —
+/// not an invented color, the panel's own background family reused for
+/// visual consistency with the rest of the window.
+/// </para>
+///
+/// <para>
+/// <see cref="UiNineSlicePanel"/> itself was not reused here — its
+/// constructor forces <c>Draggable</c>/<c>Resizable</c>/<c>Anchors.None</c>
+/// (top-level-window machinery this footer strip must never have) and
+/// draws a full 8-piece bevel + resize grip this borderless field
+/// explicitly does not want (the issue calls for "no border").
+/// </para>
+/// </summary>
+public sealed class UiSolidSpriteFill : UiElement
+{
+    public uint SpriteId { get; init; }
+    public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
+
+    public UiSolidSpriteFill()
+    {
+        // A backing field must never intercept pointer events meant for
+        // whatever's stacked in front of/behind it (the buttons draw ON
+        // TOP via their own later ReadOrder; nothing should route to this
+        // leaf at all).
+        ClickThrough = true;
+    }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        var resolve = SpriteResolve;
+        if (resolve is null || SpriteId == 0u) return;
+        var (tex, tw, th) = resolve(SpriteId);
+        if (tex == 0 || tw == 0 || th == 0) return;
+        // Force OPAQUE — an occluding backing field must read solid even
+        // when it sits inside a window whose own chrome/content is
+        // translucent (same "force opaque" pattern UiMenu's popup uses).
+        ctx.PushAlphaAbsolute(1f);
+        try
+        {
+            ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, Width / tw, Height / th, Vector4.One);
+        }
+        finally { ctx.PopAlpha(); }
+    }
+}