Render: bounded opaque surface contract and numerical BRDF reference (#1019)

A renderer-independent OpaqueSurface (colour, metallic fraction, roughness, dielectric IOR) with bounded inputs, an original GGX/Smith/Schlick reference response with Lambert diffuse, and exact piecewise sRGB8 decoding. Numerical contract only: no document format, command or UI change.

crates/render/src/lib.rs

@@ -9,6 +9,7 @@ mod camera;
 mod curve_display;
 pub mod lighting;
 mod raster;
+pub mod surface;
 
 pub use backend::{
     Availability, BackendDescriptor, BackendId, ExecutorDescriptor, ExecutorId, MAX_RENDER_ID_BYTES, RenderFeature, RenderFormat, RenderLimits,