UiElement.MaxHeight + ResizableEdges mask; UiCollapsibleFrame snaps height to the nearer
of {collapsed,expanded} and toggles row-2 visibility; GameWindow computes the two heights
from the layout + top-anchors the content. Amends IA-17. UiNineSlicePanel unsealed to
allow subclassing.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
