feat(D.2b): UiDatElement — generic per-drawmode element renderer

Generic fallback widget for every LayoutDesc element type without a
dedicated behavioral widget (chrome corners/edges, drag bars, resize grips).
Holds an ElementInfo + active-state name; draws that state's media by tiling
(UV-repeat on both S+T axes, matching ImgTex::TileCSI). DrawMode constants
documented per format spec §6 (Undefined=0, Normal=1, Overlay=2,
Alphablend=3 — no Stretch mode). Plan 1: all modes render as the same
alpha-blended tiled quad; per-mode branches deferred to Plan 2.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

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+using System;
+using System.Numerics;
+
+namespace AcDream.App.UI.Layout;
+
+/// <summary>
+/// Generic dat element: draws its active state's media by DrawMode (Normal=tile,
+/// Alphablend/Overlay=blended overlay). The fallback renderer for every element type
+/// without a dedicated behavioral widget (chrome corners/edges, drag bars, resize grips);
+/// faithful because retail's base element render is exactly "stamp the media per draw-mode".
+///
+/// <para>
+/// For Plan 1, all observed draw modes produce the same alpha-blended tiled quad — the
+/// sprite shader already alpha-blends, so no per-mode branch is needed here. The named
+/// constants document the real enum for Plan 2.
+/// </para>
+///
+/// <para>
+/// DrawModeType (DatReaderWriter.Enums), stored as int in <see cref="ElementInfo"/> to
+/// keep this dat-free. See docs/research/2026-06-15-layoutdesc-format.md §6:
+/// <c>Undefined=0, Normal=1, Overlay=2, Alphablend=3</c>. There is no Stretch mode.
+/// </para>
+///
+/// <para>
+/// Tiling uses UV-repeat on BOTH axes (<c>Width/tw</c>, <c>Height/th</c>) so vertical
+/// chrome edges (e.g. a 5×10 sprite drawn over a 5×48 rect) tile vertically too.
+/// <see cref="AcDream.App.Rendering.TextureCache.UploadRgba8"/> sets
+/// <c>GL_REPEAT</c> on both S and T, so vertical tiling is always active.
+/// </para>
+/// </summary>
+public sealed class UiDatElement : UiElement
+{
+    // DrawModeType enum values from DatReaderWriter.Enums.
+    // See docs/research/2026-06-15-layoutdesc-format.md §6.
+#pragma warning disable IDE0051 // private constants kept for documentation / Plan 2
+    private const int DrawUndefined   = 0;
+    private const int DrawNormal      = 1;
+    private const int DrawOverlay     = 2;
+    private const int DrawAlphablend  = 3;
+#pragma warning restore IDE0051
+
+    private readonly ElementInfo _info;
+    private readonly Func<uint, (uint tex, int w, int h)> _resolve;
+
+    /// <summary>Which state name to render. <c>""</c> = the unnamed DirectState.
+    /// Falls back to DirectState if the named state is absent.</summary>
+    public string ActiveState { get; set; } = "";
+
+    /// <param name="info">Merged <see cref="ElementInfo"/> for this element.</param>
+    /// <param name="resolve">Dat file-id → (GL texture handle, native px width, native px height).
+    /// Returns (0,0,0) when the texture is not yet uploaded.</param>
+    public UiDatElement(ElementInfo info, Func<uint, (uint tex, int w, int h)> resolve)
+    {
+        _info = info;
+        _resolve = resolve;
+        ClickThrough = true; // generic decoration; behavioral widgets opt back in
+    }
+
+    /// <summary>
+    /// Returns the (File, DrawMode) for the current <see cref="ActiveState"/>,
+    /// falling back to the DirectState (<c>""</c> key) if the named state is absent.
+    /// Returns (0, 0) if neither exists.
+    /// </summary>
+    public (uint File, int DrawMode) ActiveMedia()
+        => _info.StateMedia.TryGetValue(ActiveState, out var m) ? m
+         : _info.StateMedia.TryGetValue("", out var d) ? d
+         : (0u, 0);
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        var (file, drawMode) = ActiveMedia();
+        if (file == 0) return;
+
+        var (tex, tw, th) = _resolve(file);
+        if (tex == 0 || tw == 0 || th == 0) return;
+
+        // Normal → TILE at native size on both axes (UV-repeat; GL_REPEAT-wrapped UI texture),
+        // matching ImgTex::TileCSI. Overlay/Alphablend are the same blit with a blend state; the
+        // sprite shader already alpha-blends, so the quad is identical for all draw modes in Plan 1.
+        // (No Stretch mode exists in DatReaderWriter.Enums.DrawModeType.)
+        // drawMode is not yet branched here — Plan 2 can add per-mode behavior if needed.
+        _ = drawMode; // suppress unused-variable warning until Plan 2 adds per-mode branches
+        float u1 = Width / tw;
+        float v1 = Height / th;
+        ctx.DrawSprite(tex, 0, 0, Width, Height, 0, 0, u1, v1, Vector4.One);
+    }
+}

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

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+using AcDream.App.UI.Layout;
+namespace AcDream.App.Tests.UI.Layout;
+
+public class UiDatElementTests
+{
+    [Fact]
+    public void ActiveMedia_PrefersNamedStateOverDirect()
+    {
+        var info = new ElementInfo();
+        info.StateMedia[""] = (0x06000001, 1);          // DirectState (DrawMode Normal=1)
+        info.StateMedia["ShowDetail"] = (0x06000002, 3); // named (Alphablend=3)
+        var e = new UiDatElement(info, _ => (0, 0, 0)) { ActiveState = "ShowDetail" };
+        Assert.Equal(0x06000002u, e.ActiveMedia().File);
+        e.ActiveState = "";
+        Assert.Equal(0x06000001u, e.ActiveMedia().File);
+    }
+}