Driver-Seegmiller backward-facing step

Model the air volume (flow passage) of the Driver-Seegmiller backward-facing-step channel for the parallel-roof (0 degree divergence) case described in the attached NASA validation page, as ONE solid suitable for CFD meshing. Use the documented ratios and the step height H = 0.5 in = 12.7 mm: step height to tunnel exit (downstream) height 1:9, step height to tunnel width 1:12, so the upstream channel height is the downstream height minus H. Cover exactly the documented computational domain x/H = -105 to +50. Frame: +x streamwise with x = 0 at the step face (upstream is negative x), +y spanwise across the tunnel width with y = 0 at one side wall, +z vertical with z = 0 at the downstream floor (the upstream floor therefore sits at z = H). Expose H, the width, the two channel heights, the roof angle, and the upstream/downstream domain lengths as named parameters, and name the flow-boundary faces (inlet, outlet, roof, upstream floor, step face, downstream floor, side walls) in the contract so the mesher can find them. The measured reattachment location is a downstream validation target, not a geometry input; do not tune anything to it.