PDF underlays: attach dialog with pages, underlay tab, layers, import settings and file import, unload on the definition

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/model/mod.rs

@@ -6,6 +6,7 @@ pub mod image_model;
 pub mod instance_model;
 pub mod material_model;
 pub mod ole_pres;
+pub mod pdf_layers;
 pub mod pdf_raster;
 pub mod pdf_vector;
 pub mod mesh_model;