nx_layout_block_test.nx

nx_layout_block_test.nx -- smoke for the Phase-3 block-layout primitive. Builds a 2-box tree (root + child) with explicit cascade declarations on the child (width 100px, height 50px, margin-top 20px, padding-top 10px), runs the normal-flow layout pass, asserts each box's (x, y, w, h). Layout math (Phase 3 content-box subset): root: x=0, y=0, w=viewport(800), h=80 (no own padding; content_h = children_h = child's outer_h) child: x=0 (parent's content x), y=20 (margin-top), w=100 (explicit), h=60 (padding-top 10 + content 50) child outer_h = mt(20) + h(60) + mb(0) = 80