DWF and DGN underlays: attach, display, snaps, frames and references

DWFATTACH / -DWFATTACH and DGNATTACH / -DGNATTACH attach a DWF / DWFx sheet
or a DGN (V7 or V8) model with the reference's prompts; ATTACH routes the
file types to them. The sheet or model is read into vector paths, drawn as a
transparent underlay raster (contrast, fade, monochrome and background
adjustment apply) and offers its geometry to object snaps. Definitions are
kept in ACAD_DWFDEFINITIONS / ACAD_DGNDEFINITIONS, the External References
palette lists and reloads them, and image textures get line-keeping mip
levels so thin underlay lines stay visible when zoomed out.

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

src/app/commands/pdf_import.rs

@@ -218,14 +218,16 @@ impl OpenCADStudio {
         &mut self,
         i: usize,
         label: String,
+        kind: codec::entities::UnderlayType,
         path: &str,
         pages: Vec<(String, codec::EntityType)>,
     ) {
         let pending = self.begin_undo(i, label, pages.len(), false);
 
         for (page, mut entity) in pages {
-            let definition = crate::modules::insert::pdf_attach::ensure_pdf_definition(
+            let definition = crate::modules::insert::pdf_attach::ensure_underlay_definition(
                 &mut self.tabs[i].scene.document,
+                kind,
                 path,
                 &page,
             );