nx_paint_walk_layout_test.nx

nx_paint_walk_layout_test.nx -- end-to-end integration smoke for the bits-up rendering pipeline. Builds a layout tree by hand (since nx_layout_from_dom is queued), populates a cascade with background-color declarations per box, walks the tree to paint, and verifies framebuffer pixels match the expected coloring. Tree: root at (0, 0, 32, 32) with background-color: #00ff00 (green) child at (4, 8, 16, 12) with background-color: #ff00ff (magenta) Expected rendering (document-order paint: root first, child OVER): pixel (1, 1) -> green pixel (10, 10) -> magenta (inside child) pixel (28, 28) -> green (outside child but inside root) pixel (4, 8) -> magenta (top-left of child) pixel (19, 19) -> magenta (bottom-right of child = 4+15, 8+11)