Function extract_surface_style_specular 

pub fn extract_surface_style_specular(
    surface_style_id: u32,
    decoder: &mut EntityDecoder<'_>,
) -> Option<SpecularMaterial>

Extract the metallic/roughness pair from an IfcSurfaceStyle’s first IfcSurfaceStyleRendering, walking its Styles list (attr 2) exactly as extract_surface_style_colors does, so the two never disagree about which rendering “wins” for a multi-style IfcSurfaceStyle.

Lives here, in the canonical styling module, rather than as a parallel path (see AGENTS.md “Colour and coordinate resolution is canonical Rust”): both the server (process_geometry) and the viewer (process_geometry_batch) reach this through the shared GeometryStyleInfo the prepass builds (prepass.rs), so they cannot drift on the mapping.