nx_vessel_csg_test.nx

nx_vessel_csg_test.nx -- smoke for nx_vessel_csg (C1). Proves the vessel is a correct hollow cup via cheap field-sampling + inside-count (no OOM-prone meshing): - the cavity is hollow (1), the wall is solid (2), the base is solid (3), outside is empty (4) - the hollow cup contains strictly less material than the solid cylinder it was cut from (5-6) Exit code = failed assertion number; 0 = all pass.