test(D.2b): UiDatElement — cover DrawMode passthrough + media fallbacks

- Assert DrawMode values (not just File) in the existing named-vs-direct test
- Add ActiveMedia_NoMedia_ReturnsZero: empty StateMedia → (0,0)
- Add ActiveMedia_MissingNamedState_FallsBackToDirect: absent named key → DirectState
- OnDraw: replace `var (file, drawMode) = ...; _ = drawMode;` with idiomatic `var (file, _) = ...`
- Add `// exposed for unit testing` comment above ActiveMedia()

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

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

@@ -61,6 +61,7 @@ public sealed class UiDatElement : UiElement
     /// falling back to the DirectState (<c>""</c> key) if the named state is absent.
     /// Returns (0, 0) if neither exists.
     /// </summary>
+    // exposed for unit testing
     public (uint File, int DrawMode) ActiveMedia()
         => _info.StateMedia.TryGetValue(ActiveState, out var m) ? m
          : _info.StateMedia.TryGetValue("", out var d) ? d
@@ -68,7 +69,7 @@ public sealed class UiDatElement : UiElement
 
     protected override void OnDraw(UiRenderContext ctx)
     {
-        var (file, drawMode) = ActiveMedia();
+        var (file, _) = ActiveMedia();
         if (file == 0) return;
 
         var (tex, tw, th) = _resolve(file);
@@ -78,8 +79,7 @@ public sealed class UiDatElement : UiElement
         // 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
+        // DrawMode is not yet branched here — Plan 2 can add per-mode behavior if needed.
         float u1 = Width / tw;
         float v1 = Height / th;
         ctx.DrawSprite(tex, 0, 0, Width, Height, 0, 0, u1, v1, Vector4.One);