The preview rectangle used c1.y (constant) for all four corners and
pt.z for Z instead of c1.z. Corners 3 and 4 now correctly use pt.y
(varying Y) and the constant c1.z, producing a proper XY-plane rectangle.
Fixed in both layout/mview.rs and manage/mview.rs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>