nx_loop_subdiv_gate.nx

nx_loop_subdiv_gate.nx -- proves LOOP SUBDIVISION is a real SMOOTH-LIMIT scheme, not mere refinement. An octahedron (R on each axis) is subdivided once by Loop AND once by linear midpoint (sd_subdivide). Loop REPOSITIONS the poles to the smooth limit (5R/8, hand-exact); linear KEEPS them at R (same silhouette). The neg-control is the linear scheme itself: max radius drops under Loop, stays put under linear = Loop smooths. Converges over a 2nd level. Emits an STL artifact. license_tier: ORIGINAL expect_exit: 0