Unified city surfaces

City and Districts submit their solid boxes to one shared builder. Ground surfaces extend to a common bed, so the union removes covered terrain beneath roads, water, sand, parking and buildings across district boundaries. A spatial index clips buried faces; exposed rectangles subdivide at 8m for affine UVs and vertex fog. Materials produce one batch each. Main scene calls begin(), adds both districts, then finish(build, parent). This replaces the earlier independent geometry batches that left broad ground overlap.