feat(vtank): slice 7 fix — <menu style> selects plain vs retail art

Wires the new UiMenu.RetailButtonArt switch (previous commit) into
plugin markup: <menu style="plain"> (also the default when the
attribute is absent) builds RetailButtonArt=false so a plugin's
dropdown gets the flat VTank-matching box; style="retail" opts a
panel back into the gold pushbutton face. Any other value throws
FormatException at Build naming the element, matching the existing
<icon iconkind> validation convention (ValidateIconKind).

Mutation check: temporarily stubbed ValidateMenuStyle to always return
true (as if the switch didn't exist) — 3 of the 4 new
MarkupDocumentTests.Menu_* tests failed exactly as expected
(Menu_NoStyleAttribute_DefaultsToPlain_RetailButtonArtFalse,
Menu_StylePlain_Explicit_RetailButtonArtFalse,
Menu_UnknownStyle_ThrowsFormatException_NamingTheElement); the
style="retail" test passed trivially either way, as expected for that
case. Restored before committing.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/AcDream.App/UI/MarkupDocument.cs

@@ -461,6 +461,7 @@ public static class MarkupDocument
                 Func<string?> menuSelected = BindString(
                     (string?)el.Attribute("selected"),
                     binding);
+                bool menuRetailButtonArt = ValidateMenuStyle((string?)el.Attribute("style"));
                 var menu = new UiMenu
                 {
                     Left = F(el, "x"),
@@ -480,6 +481,7 @@ public static class MarkupDocument
                     PopupBgSprite = 0x0600124Cu,
                     ItemNormalSprite = 0x0600124Eu,
                     ItemHighlightSprite = 0x0600124Du,
+                    RetailButtonArt = menuRetailButtonArt,
                     ButtonLabelProvider = () => menuSelected() ?? string.Empty,
                     OnSelect = payload =>
                     {
@@ -633,6 +635,25 @@ public static class MarkupDocument
                 $"{context} must be did, spell, or item (got \"{other}\")"),
         };
 
+    /// <summary>
+    /// Owner live-client report 2026-09-07 ("Those BIG gold/yellow buttons HAS
+    /// to go. That is not how vtank looks."): validates <c>&lt;menu
+    /// style="..."&gt;</c> and returns the <see cref="UiMenu.RetailButtonArt"/>
+    /// value it selects. Default (attribute absent, or explicit
+    /// <c>style="plain"</c>) is the flat VTank/Decal <c>HudCombo</c> box
+    /// (<c>false</c>) — retail's gold pushbutton art is now an explicit
+    /// <c>style="retail"</c> opt-in for a plugin panel that genuinely wants
+    /// it. Any other value is a Build-time author error, same rule as
+    /// <see cref="ValidateIconKind"/>.
+    /// </summary>
+    private static bool ValidateMenuStyle(string? style) => style switch
+    {
+        null or "plain" => false,
+        "retail" => true,
+        var other => throw new FormatException(
+            $"<menu style=\"{other}\"> must be plain or retail"),
+    };
+
     /// <summary>
     /// Builds the zero-argument icon resolver the <c>&lt;icon&gt;</c> element
     /// uses: dispatch by <c>iconkind</c> (default <c>"did"</c>) to the

tests/AcDream.App.Tests/UI/MarkupDocumentTests.cs

@@ -314,4 +314,58 @@ public class MarkupDocumentTests
         Assert.Equal(0.78f, binding.AttackPower);
         Assert.Equal(0.78f, slider.ScalarPositionSource!());
     }
+
+    // ── S7 fix ("BIG gold/yellow buttons has to go" — owner live-client
+    // report 2026-09-07): <menu style="..."> selects UiMenu.RetailButtonArt.
+
+    private sealed class MenuStyleBinding
+    {
+        public IReadOnlyList<string> Choices => ["First", "Second"];
+        public string Selected { get; } = "First";
+    }
+
+    private static string MenuXml(string? styleAttribute) =>
+        "<panel x=\"0\" y=\"0\" w=\"240\" h=\"60\">" +
+        $"<menu x=\"4\" y=\"4\" w=\"120\" h=\"20\" items=\"{{Choices}}\" selected=\"{{Selected}}\"{styleAttribute}/>" +
+        "</panel>";
+
+    [Fact]
+    public void Menu_NoStyleAttribute_DefaultsToPlain_RetailButtonArtFalse()
+    {
+        var panel = MarkupDocument.Build(MenuXml(styleAttribute: ""), new MenuStyleBinding(), _ => (1u, 32, 32));
+        var menu = Assert.IsType<UiMenu>(panel.Children[0]);
+
+        Assert.False(menu.RetailButtonArt);
+    }
+
+    [Fact]
+    public void Menu_StylePlain_Explicit_RetailButtonArtFalse()
+    {
+        var panel = MarkupDocument.Build(
+            MenuXml(" style=\"plain\""), new MenuStyleBinding(), _ => (1u, 32, 32));
+        var menu = Assert.IsType<UiMenu>(panel.Children[0]);
+
+        Assert.False(menu.RetailButtonArt);
+    }
+
+    [Fact]
+    public void Menu_StyleRetail_OptsIntoTheGoldButtonArt()
+    {
+        var panel = MarkupDocument.Build(
+            MenuXml(" style=\"retail\""), new MenuStyleBinding(), _ => (1u, 32, 32));
+        var menu = Assert.IsType<UiMenu>(panel.Children[0]);
+
+        Assert.True(menu.RetailButtonArt);
+    }
+
+    [Fact]
+    public void Menu_UnknownStyle_ThrowsFormatException_NamingTheElement()
+    {
+        var ex = Assert.Throws<FormatException>(
+            () => MarkupDocument.Build(
+                MenuXml(" style=\"chrome\""), new MenuStyleBinding(), _ => (1u, 32, 32)));
+
+        Assert.Contains("menu", ex.Message);
+        Assert.Contains("chrome", ex.Message);
+    }
 }