refactor(render): unify scene rendering paths

Route model, paper, viewport and nested block content through one traversal while preserving leaf renderers. Carry decoded material state into mesh shading and harden solid geometry conversion.

src/scene/model/material_model.rs

@@ -124,7 +124,7 @@ impl MeshMaterial {
             description: String::new(),
             diffuse: color,
             ambient: [color[0], color[1], color[2]],
-            specular: [1.0; 3],
+            specular: [0.08; 3],
             gloss: 0.5,
             reflectivity: 0.0,
             self_illumination: 0.0,
@@ -169,18 +169,11 @@ impl MeshMaterial {
         let bump_map = MeshTextureMap::from_dwg(&material.bump_map, base_dir);
         let refraction_map = MeshTextureMap::from_dwg(&material.refraction_map, base_dir);
         let normal_map = MeshTextureMap::from_dwg(&material.normal_map, base_dir);
-        // A material whose diffuse channel explicitly references an external
-        // texture is incomplete when that file cannot be resolved. Its stored
-        // diffuse RGB is commonly only a red/magenta placeholder (for example
-        // VILLA_COMPLETE's Plaster material), not the intended appearance.
-        // Keep the material's other physical properties, but use the entity's
-        // fully resolved Color/ByLayer/ByBlock value for the missing diffuse
-        // channel.
-        let diffuse = if diffuse_map.has_content() && diffuse_map.image.is_none() {
-            entity_color
-        } else {
-            material_color(&material.diffuse_color, entity_color)
-        };
+        // The stored diffuse component remains authoritative when an external
+        // image is unavailable. Replacing it with the entity colour would lose
+        // a decoded material value and make the result depend on a guess about
+        // the missing asset's intended appearance.
+        let diffuse = material_color(&material.diffuse_color, entity_color);
         let ambient = material_color(&material.ambient_color, entity_color);
         let specular = material_color(&material.specular_color, [1.0; 4]);
         let opacity = material.opacity_percent.clamp(0.0, 1.0) as f32;