4-task TDD plan: UiItemList grid mode (CellOffset/Columns/cell pitch),
the LayoutImporter.Resolve sub-window mount (ShouldMountBaseChildren +
attach base subtree), GameWindow swap of the placeholder for the real
Import(0x21000023), then full-suite regression guard + visual.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
