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>
19c831211b