Stage 3 — materials. One rule per family (notes/direction.md): glass transmission 1 / IOR 1.5 / roughness 0.04; brushed steel metalness 0.9 / roughness 0.34 / anisotropy 0.6; powder coat roughness 0.62 / clearcoat 0.18; rubber roughness 0.88; walnut roughness 0.52 / clearcoat 0.35. One shader patch: machined-groove roughness and edge glint on the burrs. Lighting and camera unchanged.
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